Sorry if I mislead or was unclear, I've never owned the CMD 121P combo though I've really thought about it. I have used one in a gig environment a couple of times and tested another @ GC extensively when I was initially shopping for combos - ended up with the 102P. With that in mind though, I can tell you that the 102P offers a fuller sound and was perceived as louder. The 121P is no slouch though and I doubt there's much ground that the 102P can cover that the 121 can't. Tonewise, they are very similar in that they both have the smooth Markbass tone. The 102P again, being a more full sound and seeming to have just a tad more bite up top than the 121P. The 121P with a New York 121 extension cab is phenominal. While the 121P is great by itself, when paired with the extension you'll be blown away. The 102P, when I tested them, benefitted from the addition of a traveler 102P cab as well but, I thought the 121P really opened up with the extension cab. With a little adjustment to the VLP filter, I think you can get real close to an Ampeg type sound if that's what you're looking for. Add a VT bass to your chain and you're definitely in the ballpark. Me? I was looking for something more transparent and use mine set relatively flat with just a touch of VLP filter added.
I'd venture to say that at the same volume in a band/gig environment you'd be hard-pressed to tell the difference between the two. The deciding factor for me came down to the 102P being a kickback style combo. That's what I wanted and was willing to add a few pounds and a bigger overall cab to get that. Both are really great IMHO and it'd be hard to wrong with either.
I'll also tell you that when I bought the 102P combo I had every intention of getting an extension cab....but, I never needed it. I came from the big 8x10 setup and was doubtful that the little combo could keep me in the same comfort zone volume-wise that I was used to with the 8x10. While I'm not knocking that setup, I've never looked back and would not anticipate going back to something that big - unless, I have someone to haul that stuff around for me. The kickback design also proves very beneficial in being able to hear yourself although, admittedly, there are stands and other devices that can tilt your cab back or bring them up further for better hearing.
If you don't mind the extra 16 or so pounds of the CMD 102P, it's the front runner for me. However, at 28 pounds, if you like the Markbass sound, the CMD 121P is the way to go if you're looking for a compact and lightweight combo that can really knock your socks off