I am nowhere near the level of guys like trigggl & Slicker, but I think I can answer your question.

Ubuntu - Easy to install, has a lot of features and good online support. But because it has a lot of "stuff" it is also FAT.

Gentoo - Very fast, lean and highly configurable. In order to do this Gentoo has to be compiled from source code, built from scratch so to speak. You have to "tell" Gentoo what you want it to be. This requires knowledge, effort and time. Gentoo also has good online support but it is geared more towards knowledgeable users.