I really like the Klingon rifle, too. Essentially, it's just a new stock added to the standard single-handed disruptor pistol for a two-hand hold and, presumably, extra power or features.
Also, the new stock can apparently be bolted onto other types of weapon as well. In "Too Short a Season", the wall of Governor Karnas features various guns, including one fashioned from the Klingon stock and a more compact forward end; this type of gun is used to good effect against the E-D personnel.
Of the Starfleet rifles, I'm still partial to the original TNG one. It's more innovative than the ST:FC ones: it dares to look different, rather than imitating current gunpowder-based designs, and the size-to-power ratio is at least somewhat reasonable. It makes no sense to build a rifle as big as the ST:FC ones if it does little more than the Type 1 palm weapon.
Probably the ST:FC rifles do have some added benefits, though - and not just over the palm and hand phasers, but over the "standard" TNG phaser rifle as well. After all, in DS9 "Empok Nor" it appears that the ST:FC rifle that Nog totes is supposed to be a more powerful weapon than the regular rifles carried by the security detail. Basically, then, the TNG gun could be an "assault rifle" or a "submachine gun" while the ST:FC weapon could be a "light machine gun".
You don't send your entire platoon out armed with M-60s in normal conditions; a combination of lots of M-16 and a single M-60 works better, as in "Empok Nor". But if your ship is being invaded by the Borg, then perhaps it does make sense to hand out M-60s to every man and woman...
Timo Saloniemi