the main reason i use it is because it's updated, and the mods have been changed/added more features in the later versions and are generally less glitchy and less overpowered. notably advanced computers/monitors in computercraft, a bunch of nice blocks in RP, decorative and functional, a couple of useful tools, more farming options, and xtrabiomes, which is nice, despite how i've been bitching about my stuffty world. the creative menu is a lot better too.
plus how there are probably more mods in FTB than there are in tekkit. last i checked, FML read 36 mods on tekkit and 53 in ftb.
and yeah, FTB has the portal gun / gravity gun mod, so