Put yourself in my shoes for a bit. You've done some T4/T5 content in TBC and you've done Naxx 10 (with a terrible guild, that struggled on Kel) and a bit of Ulduar 25 in WotLK. You haven't got much experience in heroic modes or raiding in Cata. (I chose to play more casually this expansion, focusing on PvP) But you you feel you are a good player that can handle most raids and perhaps even some heroic content, with practice.
You've sent in an app for two guilds, and been accepted by both. Guild 1, on first glance is a bit fast and loose with everything. They have cleared heroic DS and they plan for me to be in group 2. I don't know much about them and they aren't that friendly, or better put, I'm not sure my position in the raid would be solid, they are also Level 25. Guild 1 are level 8, and a group of freinds. They have cleared normal DS, and only some have cleared heroic ds. They say I'm guaranteed a raid spot in 10 man, and they might shoot for heroic raids later on.
So, I'd very much appreciate what you would pick. It's a fairly clear stereotypical distinction, one being the bigger and better guild, but with less of a chance of me being kept there, or a smaller guild where I'm apparently 1 of 2 melee's in a 10 man guild, that don't progress as much but is obviously more stable.