Agreed. the visuals aren't really important but the buffs and internal remodulator from the Mark X sets come in very handy for a novice STF operative.
While the Omega suit has less protection it does have a buff that gives +62 or so energy damage resistance and a stealth benefit. The carbine by itself isn't great compared to my new fun toy a Mk 12 anti Borg Disruptor Minigun which can mow down an entire group of drones in secondary fire mode. Sure you remodulate after every (gigantic) burst but as a street sweeper this thing is more effective than auto rifles or SMGs.
It's more or less a facet of game design that doesn't give any benefits to a handy little SMG, if you can use heavy weapons (minigun) with no penalties in close range combat.
MACO suit shield capacitor might be useful for an Engineer char because if you say, need to solo tank bosses then you need all the regenerative ability you can get.
Despite the relatively low damage resistanceI tend to use the Jem'Hadar suit when starting off a toon's STF career as the self-heal from the full set is very useful for Tacs. However it doesn't have any physical damage resistance so you can't melee tank Armek with it.
End of the day I will always use the Omega set for the sleek / utilitarian aesthetics. I'm not very keen on the "Power Rangers" look of the MACO suit.