Sidious stomps. Make it full gauntlet Thanos and then it becomes a game of who hits first, assuming that the gauntlet can kill someone as powerful as Sidious as it did appear to have limits (the snap ruined it, at least temporarily). However, seeing as the snap itself did kill half the life in the universe, I'd say it's fair to think it possesses more power than Sidious does, unless we appeal to the all of space quote.
If Thanos did succeed in killing Sidious, there'd still be the possibility that Sidious would return as a spirit. We don't know the precise details of how the gauntlet and the stones work in the MCU, so it's hard to say whether Sidious' spirit would go into the soul stone or just cease to exist completely. If the former, then there's a possibility he could escape the soul stone and return to fight Thanos as he did abscond from Chaos several times.
On the other hand, Sidious can still just blitz Thanos or one-shot him in a variety of ways, seeing as Stormbreaker could power through his energy beam and pierce his chest armor, leaving Thanos heavily wounded.