Go for gog, steam prices will probably be matched or lower during the winter sale.
That's really not the way it is with sales at all on GOG. Steam Sales tend to be better than the ~50% GOG tends to offer during the holiday sales. Not to mention the games that have packs on Steam (more discounts), something GOG still tends to lack.
I'd wait to see what Steam has to offer on the sales, if there's anything there you're looking forward to, although, if you're after the Indie games I doubt Steam will be offering a similar deal at the moment (although they sometimes have Indie bundles) . Still, it doesn't hurt to wait till the last day with sales. No rush to pick them up.