They aren't very different. It's not a big difference like JKEdit and JED. GTK Radiant is a modified version of Q3 Radient, and is superior. It has a better layed out GUI, and has a few easier features. Q3 Radiant downloads are usually missing files, and half the things in the editor never worked for me. You definatly want GTK Radiant to learn Q3 editing. You can get it at http://www.qeradiant.com
For tutorials, I'd suggest:
You'll find Q3 mapping VERY easy if you've had JED experiance. The change from a sector-based game engine (JK) to a brush-based game engine (Q3) is a little wierd, but once you get the hang of it, it's VERY easy.
I know I've found myself limited when doing Q3 editing...for example, you can't make lights flicker, or doors swing without scripts, and Q3 has no scripting language for levels (that I know of), sort of like COG for JK. However, I'm almost completely confident that JO will have this. Why? RtCW has it, EF has it, Alice has it, FAKK2 has it, MoH:AA has it...they are all games that need it for the single player puzzels and AI. Q3 doesn't need it, tho, since it's just arena fighting. That's why I'm almost sure JO will have it.
*gasp* ... *gasp*