The Tavern

Is it better (18 UC) to
a) stay without gloves since base damage for UC would be decreased
or
b) get the +5 slaying (and AC as a bonus) ?

Short answer: (b).

Long answer: Depends on your Str and fighting, but with non-astronomical values you'll do more damage with the gauntlets. The claws give you 2 more base damage; multiply that by <your Str + 29>/39 x <your Fighting + 60>/60 for a total expected marginal increase. At Str 30 and Fighting 20, that comes out to just over +4, one less than the gauntlets' +5. And this is to say nothing of their boosts to accuracy or--especially--AC.

My reading of the aux attack page on badwiki is that at high UC, your offhand punch is better in a glove than with claws... that can't be right?

http://crawl.chaosforge.org/Auxiliary_attack

I think the claws/blade hands lines are meant to describe bonuses applied to the 5 + UC/2 base offhand punch damage.