I've got one of Dons two tens loaded with B & C drivers and it kills. I suspect a 210 or 310 might be a good match for a couple of reasons.
1) It's going to give you a couple of different flavors at smaller gigs. The 15/6 covers old school real well and the 310 will give you some Marcus sizzle if you want.
2) I find that sometimes mix/matching cabs or even heads sometimes makes for great combos. For instance a big wider sounding head will sometimes sound better with a tight punchy cab. I suspect that two different type cabs could give you the best of both worlds.
The only issue with a 310 will be impedance. The 310 will likely be 5.3 ohms or some other odd match, so you might want to check if your head can drive the mis-matched cabs. Your also going to need a calculator to figure out what your impedance is when your running them together.
