I already have a DB115 (8 ohm) and would like to have a second cab to make for a stack- one for home and one to keep at my practice facility. For the second, do most recommend to get the same cab or mix it up? Is there any benefit to having a 115 and a 210 or should I go for two 115s? I'm going through a TH350, if it makes any difference.
If you like your DB115, then get another one. Identical pairs work best together. Any benefit (volume/displacement) from a 115/210 wouldn't be as great that from a 2 x 115. And a 115/210 (any non-identical cabs) would result in phase & dispersion issues. Would these issues be noticeable? Would they sound bad? Maybe not. But maybe. You'll know when you hook'em up. These issues are avoided altogether with identical boxes. If you don't care for the DB115, sell it & pick up something you do like. Then, if you need/want more volume/displacement, double up on that cab.
+1 a pair of DB115's would be solid, and would play well together. Anything else would offer less, not more.