Both Direwolf20 and Mindcrack have complete FTB worldgen, so you can always start a world and change packs later.
Mindcrack also has Traincraft which adds additional worldgen but it's unclear if this mod will be unique to the Mindcrack pack or not at this time.
For beginners to Minecraft modding, I recommend the Direwolf20 pack as it's the best showcase available for quality mods. This will probably stay true even when additional FTB packs are released in the future.
Edit: Completely forgot about GregTech's extensive worldgen, I am a moron, see below.