I've had several rigs over the years with 10's, 15's and 18's. My experience has been that the 10's deliver a more punchy, focused sound. Another way to describe this is to say the sound is more defined and allows the player to be more articulate without muddiness.
Most of my experience has been with the Ampeg SVT Classic with a Ampeg HLF 6X10 cab, David Eden with an Ampeg HLF 4X10 and 1X15 cabs and MarkBass 4X10 as described below. I would rate them:
1. MarkBass
2. Ampeg
3. David Eden
I currently play MarkBass CMD 102 P with an additional cab, the STD 102 HF. This gives me 4X10 and I think it's hard to beat. I use the CMD 102 P by itself in rehearsal and small venue settings and it does well. But my ears tell me the best tone and punch is delivered with the 4X10 set up.
If you can go for the extra cab, I think the set up described above is the way to go. If you can't, I'd choose the 2X10 over the 15 every time.
Plus, as you know, the MarkBass set up is quite light. The two units I use are only about 45 pounds per unit as compared to the Ampeg 6X10 rig at 90 pounds for the head and about 120 pounds for the cab....the Eden set up was about the same total weight as the Ampeg, maybe 25 pound lighter, so the MarkBass delivers a great punch and saves your back.
Our band plays roots rock, blues, a bit of country, honky tonk, rockabilly and swing. The MarkBass is great with this variety.
Hope this helps your decision. Good luck!
Koog.