Gerry, this is right up my alley! I have the 10-2 combo also, and was in the same decision-making as you about the second cab. After talking with Jeff and others, I went with a second 10-2 cabinet so the capabilities of the cabs are the same, plus it matches up visually.
If you've gigged your combo, you know how much sound that little thing puts out. With the second 10-2, you're getting twice as much. The full 350 watts and the doubling of volume with two cabs. It is a whole lot of sound. The 12-3 cab is rated to go lower, but I'm not sure in most situations it'd be a noticeable difference, especially if you're like me and try to rein in the lows with a HPF pedal or roll off a bit of the bass control.
Although I may still try a 12-3 in the future, I am very happy with the second 10-2. I play lower volume gigs and probably will mostly play the combo by itself, but I wanted the second cab to have that extra volume capability when I need it. If you have other questions, or would want to see a pic of the stack, let me know.
On a side note,
@MaxSpinrun has let me know that we actually have a Genzler dealer here in KnoxVeegas -
Lane Music - and I may be going over there this month to try out the 15-3 they have in stock. I might go with a single 15-3 for bigger gigs.. or just because I'm really curious about what that cab is like.