I would say use both as everyone has already pointed out, GOG is good for older games at cheaper prices, but their selection on newer titles is limited. Steam has a pretty vast library and their weekend sales almost always have something that interests me. You will most likely have to call your bank after your first GOG purchase though as they are based in Poland or somewhere in that area of the world and your bank will most likely decline the charge. It's a quick 5 minute phone call fix and just tell them what the site is and that you'd like future charges to go unhindered.