Actually it was a poll question so here are the replies!
Severus Snape pros
Made Dark spells as a teen
1 of the best in the world at Potions, Dark Arts, and Occlumency
He is very talented at Legilimency
Has almost nothing you can emotionally imbalance with
He arguably has the largest arsenal out of any character in the series, showing mastery of Dark Magic, light magic, and in many different branches of magic
capable of independent flight
Severus Snape cons:
He never struck me much as a dueler type. He seems to ends fight as fast as possible and is not as flamboyant as several of the Gryffindors
He probably doesn’t have the stamina for long fights, seeing as he is not very athletic
Bellatrix Lestrange pros:
Dark arts specialist
Very athletic, able to keep up with 3 skilled teenage girls at once
Very aggressive and fearless
Has a history of taking down mighty duelists, even Aurors
The only person besides Voldemort to block magic from Dumbledore himself
Bellatrix Lestrange cons:
Like her cousin Sirius, she is arrogant and will drop her guard of facing a foe perceived as weaker
Blindly focused on the immediate goal without considering other factors, like she is more focused on hurting Harry than doing the mission
Blind faith in Voldemort/the dark arts. She is so fast to follow through with her beliefs she is blind to his rare deceits and set-ups.
Dark Arts over-specialization. She only seems to be good at Dark Arts. Does she have any other mighty facets of magic?
This is very close, but I would have to give the edge to Snape. Bellatrix has no leverage over Snape, and Snape would know how to get under the non-subtle Bellatrix’s skin. There is also no spell that Bellatrix can do that Snape can't.
voted in favor of Snape: though I think the two are very similarly matched. Both learned directly from Voldemort, and I personally believe that each of them may very well have surpassed him slightly, without his knowing (for instance, Snape was able to shield his mind from Voldemort, demonstrating at the very least that he was a superior Occlumens). But I also believe that Snape learned a great deal from Dumbledore, and from the faculty of Hogwarts, and from the Order of the Phoenix; given he has been established as perhaps the most accomplished Legilimens/Occlumens known in the Wizarding World, I shouldn't find it implausible if he knew the minds of virtually everyone in the story with whom he had contact/vicinity, since he could very likely have probed their minds without their knowledge (as Harry suspected him of doing on numerous occasions).
Bellatrix may very well possess similar powers, though we are less privy to them; take for instance that she taught Draco Occlumency, sufficient to shield his thoughts from Snape, and Dumbledore as well. I submit that this demonstrates an exceptional skill and mastery of Occlumency on her part, though we are not given any instances of her using such powers herself.
Snape is the only wizard known apart from Voldemort himself to be capable of defying magical law and flying unaided using only magic; whether he was taught this by Voldemort (not impossible), learned it via Legilimency (also a possibility), or invented the spell himself (he is established as a prodigious spell-creator; it's not impossible that he invented the flying spell, and even taught it to Voldemort, rather than vice versa), this demonstrates incredible skill and magical aptitude that I think places him firmly in the same echelon of power as Voldemort himself.
Bellatrix, having been taught by Voldemort (or, more specifically, she claims to have "learned" from him; it's a possibility that this was through Legilimency), possessed a mastery of the Dark Arts that is at least equal to Voldemort's: she was capable of using the Unforgivable Curses non-verbally, and her use of the Cruciatus Curse, in particular, seems superior to her master's (hers was of such intensity that Hermione was rendered unconscious, and the Longbottoms, Order of the Phoenix members and experienced Aurors, were driven irretrievably mad). Bellatrix also displays shades of a sort of protective attitude towards Voldemort that I think borders on a kind of mother-to-child relationship. As worshipful as she was with regard to Voldemort, he was a half-blood, and she was a pure-blooded descendent of the House of Black: I have to imagine that she had an inkling of a feeling of superiority to Voldemort, if for that fact alone. At every possible instance, she was intent on killing Harry herself, despite Voldemort's objections; her protective attitude at times insinuates that she fears for her master's safety, implying a degree of distrust of his power, which could theoretically imply a sort of superiority (why would she fear for Voldemort's safety, and not her own, if he were greatly more powerful than her?).
I'm of the opinion that Bellatrix and Snape each were at least roughly equal to Voldemort, if not somewhat superior (and that may or may not apply to Dumbledore as well; Bellatrix was able to fend off Dumbledore unscathed, and Dumbledore praised Snape's abilities as remarkable and essentially world-class on numerous occasions). Were Bellatrix and Snape to duel, I imagine it might end in a stalemate; they're incredibly evenly matched. Though, again, I lean ever so slightly in Snape's favor, as he had access to the knowledge of both Voldemort and Dumbledore.
Bellatrix wins in this duel. Sure Snape invented spells in his youth. He also got those same spells used against him by James Potter. James was more than a match for Snape in Hogwarts. To be honest Voldemort just used Snape as a convenient spy. That was all Snape ever was in all 6 of the books. Bellatrix on the other hand was one of Voldemort's most feared lieutenants. She defeated a few Order Of The Phoenix members (Tonks, Kingsley, Sirius) and even toyed with Harry before Dumbledore confronted her. Dumbledore cast a spell at her which she easily deflected. She then went on to challenge and defeat several Snatchers at once including Fenrir Greyback at Malfoy Manor. The same Snatchers that managed to capture the Golden Trio. She also dueled Ginny, Hermione, and Luna at the same time. She nearly killed Ginny in that duel before Molly interfered. I will always believe the Molly- Bellatrix duel was just fanservice, plus it fit Rowling love conquers all theme. I even read she that wrote that scene as a nod to stay at home moms. A more realistic fight would have been Bellatrix killing Molly along with all 3 girls. Seriously, a woman that kills Aurors on a regular basis manages to lose to a mom that hasn't dueled years? That's like me beating a karate instructor. The fight would last for 5 seconds at best. Of course a more realistic book 7 would have had Voldemort slaughtering everyone at Hogwarts. Book 7 was rushed and poorly written with too many plot devices to help the good guys win. Suddenly Harry can spread his protection over everyone without a spell. Plot devise after plot device.