I don't think you can make a case for either, that's why I have both
I currently use the famous GK200MB combo as my ultimate light weight system for rehearsals in a choir or with a brass band. It sounds fairly good and is loud enough, And, I can have the bass on my back and the amp in my hand and just get to the rehearsal, plug in, and go.
My big system is centered an iAMP800 head and a CXL112 from Euphonic.
For gigs with the brass band and the choir I use this system because it sounds better,
I also can hook up the GK (headphone or DI out) and have a light small system that is loud enough to keep up with 2 guitars and a drummer.
For serious rock I add a 15"EV to the CXL112, for more funk sound a Trace 2103X (not the "H", the "X" with the 2x5" speakers)
And I can hook up just everyting for about 1000W.
If you try to decide, how about the following questions:
- Do you need it very loud? If no: Combo is likely easier to move than separate. Almost every company offers good combos. I would check out the EA 200W iAmp combo or the Mark Bass. I also like my Microbass combo from GK, but that's a question of taste.
If yes (you need it very loud):
Do you insist on an extremely good sound? If no you can get away with a 15" combo with 100 - 200 THAT HAS A LIMITER IN THE POWER AMP. (If you don't have that limiter its much easier to blow a speaker. Most of these combos like Yorkvilles or Peavey give a really decent sound at high volumes and are indestructible.
If you want an excellent sound at high volumes you can go with things like iAMP800 combo, but that offers in most cases no real advantage over having amp and box separate.
The problem I see with high power combos is that they start to get real heavy really quickly. For high power and good sound I went with different Head and speakers, and I don't regret it. Another advantage is that you can leave some speakers in the rehearsal room and just bring your head for rehearsal.
Oh, I should mention that I live upstairs, so I have to drag everything up one flight of stairs, thats why weight is an issue. If you live on the first floor and back up our truck into your rehearsal room weight may not be as important to you.