As a Sci-Officer and carrier captain myself... I prefer the Vo'Quv hands down.
Here are my personal reasons:
Vo'Quv has one less weapon. For giving up this weapon, it gains a hangar bay. A hangar bay is *infinitely* more valuable than a single weapon slot - and requires absolutely zero weapon power investment to to use.
To explain this in detail, 1 hangar bay can be worth up to 9 fighters or 3 birds of prey, and either of those things carry their own photon torpedoes and cannons/turrets - essentially they're multiple weapons systems all unto themselves.
The Vo'Quv *is* slow - no doubt about it; but it's also ridiculously hard to kill in the right hands, and while 6 guns isn't a lot, when you take the proper Tactical skills* you can dish out solid damage on your own while softening up targets for your fighters.
I *highly* recommend investing in at least 1 beam fore and aft to make use of Target Subsystems. I know some people think it's not worth much; but I honestly feel it's a very useful ability, and one that if used well can be just what you need to break an opponent's shields for your fighter's torpedo salvo to get through.
I'll add that while the Vo'Quv is a very tanky ship, you can make use of things like Scramble Sensors III and Jam Sensors III to make even targeting you a pain in the butt; without really detracting from your ability to take a pounding directly.
The Kar'Fi meanwhile gains a Lt level Tac Officer slot... at the cost of your Lt. Commander level Engineering power, and one of your Lt. Science powers. By itself that would be OK, but you also lose hull and crew, and while you pick up a gun... it costs you a hangar bay. As I noted above, hangar bays are much, much more valuable than guns.
Now here's the big problem with the Kar'Fi - Not only are you losing a hangar bay, but what you can actually put IN that hangar bay is not very good imo.
The S'kul fighters are, imo, a total joke. The problem is that they're main selling point is also their greatest weakness: They ram the enemy ship after awhile. Now, ramming does some reasonable damage - but it deprives you of a fighter's constant DPS, and the S'kul fighter's DPS is, as I recall, already below that of a To'Duj in a straight fight.
So you're giving up a hangar bay, and you're losing fighters not only to them getting shot down normally, but also by them ramming the enemy ship - whether it has shields up or not - ultimately making keeping a high quantity of fighters out very, very difficult.
The other option, the Fer'jai frigate is far, far superior; in fact let me just give them credit here: I like the Fer'jai quite a bit more than I do the BOP a Vo'Quv can use. That said, frigates take longer to deploy than fighters, and while their tricobalts can hurt a lot... they can also of course be shot down.
The big thing here is again... you've only got the one hangar bay. Where a Vo'Quv could have a whopping 6 BOPs... you've got, at most, 3 Fer'Jai.
And as a final insult... that extra gun you can mount takes up extra power to fire, and being a Sci-Heavy ship and a carrier (and thus your hangars are tied to Aux), you'll want your Aux at a minimum in the 50+ range; which means you're either not-maxing weapon power or losing some base shield setting. Either way, this is more painful for a ship who's loss of a hangar is supposed to be balanced by an extra weapon slot.
Now that said, I can think of one use for the Kar'fi off the top of my head - due to it's higher turn rate you can probably use an offensive Sci build more effectively.
On the one hand, this is good because other than the Varanus the Klingons really lack a ship that can do this. (The Vo'Quv is too slow for directional sci powers most of the time. You can get your Subnucleonic Beam off now and again, but even that can be tricky and requires evasive maneuvers)
On the other hand, using offensive sci powers further weakens your already not-great defenses.
Can you make a Kar'fi work? Absolutely. However it's a lot harder than the Vo'Quv, and the payoff for a well-played Kar'fi is merely 'adequacy' while the payoff for a well piloted Vo'Quv is 'supremacy'.
That is - if you fly a Kar'fi really well, you'll do fine. You won't be extraordinary or anything, but you'll do well enough for yourself. If you fly a Vo'Quv well, you can and will dominate anyone who isn't on their toes.
All of the above said, this is also all likely to change a bit - Heretic's thread on Carriers suggests that the Kar'fi may get it's second hangar bay and a lot of other love in the relatively near future. So it may be worth flying before long.
*Take Beta I or II... I recommend II myself. Delta is also good, but in a big furball where you're liable to get ignored it can be hard to get it on the right target. Of course you could take Delta I and Beta II. I also like Tac Team for the ensign slot due to the shield facing reinforcement.
Youre wrong about one thing..
A wellflown Kar'fi isnt just adequate.. Its an amazing ship, able to put some serious hurt on just about anything.. A Vo'quv is easier (just like a zombie cruiser) but is a rather passive ship.. IMO the Kar'fi is just the right blend of Offensive power, combined with just enough defenses/heals to be a pain to kill. I wish it had a Uni Lt slot in place of the Tac though.