It's a tough call either way. I've owner several 123's as well as several 124's, and unless the 124 is a basket case, I'd have a "for sale" sign on that 123 quicker than you can finish reading this sentence. Once you get the 124 running properly, you'll never want to drive (or work on) a 123 again. But, it can take some real effort to reach that point. You could always market both, at the prices you want to get, and see which sells first...