I vote for the fridge. Admittedly, I've never owned a standard kegerator or a chest freezer. If you are fermenting AND serving, I think the kegerator is out. The chest freezer might be your best option just due to space, although I've heard of people having problems with moisture collection/mold/dampness as well as cycling on and off of the motor to maintain temps appropriate for fermenting. There are of course, solutions for these issues.
I guess the big question here is how do you think is the best way to maintain all of your different tempeartures:
Serving: 40-50
Ale fermenting: 60-85
Lager fermenting: 48-55
Diacetyl rest: 68
Lagering: 30-38
So you see that any one vessel is not a solution. Even if you only fermented one beer at a time and "moved" the internal temp with a controller, you're hands are still tied by the beer that you are serving.
One solution is a ferm wrap wrapped around the carboy wrapped in a blanket. The fridge can be at a constant 40 degree serving temp while the ferm wrap can control the "environment" of the fermenting beer through all it's stages, wether it be ale or lager.
If you go with the ferm wrap solution, then it's between the fridge and the chest freezer. I still like the fridge. Specifically an over under with the fridge on top. That way the carboys are all up at tit level so you can deal with them easily.
Also, since it's a fridge, it has a freezer which can be used for storage of hops, dme, ice packs, etc. The important thing being a fridge has two "areas" where a chest freezer has only one. With a fridge you can do all of your fermenting and serving in one area while freezing/storing in the other area. With a chest freezer, it's just one total area, which will probably be kept at serving temps (40-50), so you really are losing out on owning any freezing capability. (Because why would any idiot keep the whole thing at freezing for the sake of hop storage, and THEN wrap cornies in ferm wraps up to serving temps just so they don't freeze)
So, I vote fridge, specifically one where the fridge is above the freezer.
Jason Gabriel