I have a very soft spot for Duals. One difference with the 1228 is that the platter has a smaller diameter, 10+ inches vs 12 inches with the 1229 (and 1219); this leads to these platters having different weights as well, and I believe the larger/heavier platter is preferable. Here's a link to a great reference site for Dual turntables:
Turntable Index Page It provides specs for just about every turntables Dual has manufactured over the years.
Having said all that, if the 1228 is in good shape and functioning well, I'd say it's a steal at $120, especially if it has a dust cover.
You could slip it into your second setup and I'm guessing it would be an improvement. (Or you could put it upstairs and move the 1229 into your more dedicated listening space.)
Oh, and another nice thing about a good Dual turntable is that it will handle very good cartridges, meaning that the Duals are capable of rising to the quality of the carts you put on them. If you saved a good portion of your $500 going with a Dual, you could invest in a solid cartridge with what's left. I've used several Shure cartridges on my Dual 1219, the best being a V15 Type IV with Jico SAS stylus (current). I also have used a Denon DL 103R. The Dual has handled each of them very well.