I'm very very very very very biased towards Shopsmith
But, I'll put that aside and try to answer your question.
Simply put, I don't think a midi is a good value. Before the Dark Siders start sharpening their skew chisels, I did not say midi lathes are junk or won't do a job well.
I find the midi lathe is an expensive (Delta is now in the $500 range) and undersized tool that is easily outgrown. It also is of limited use outside of turning.
For that reason, I'd suggest getting a used Shopsmith that has the lathe stuff with it. There was an old Magna era Shopsmith on Craigslist this week for $75.
Also, you get 700 to 5,200 rpm variable speed control using a dial, not changing belts. In the end, you also have a tool capable of many tasks. As a drill press and disc sander, its hard to beat.
I'd say you can safely find a well equipped Shopsmith in the $350 range. You will also find ~2.38 gazillion parted out Shopsmiths on e-bay, so I'd not let the "proprietary" issue factor too heavily in such comparisons.
Hope I kept my bias to a minimum.....
Jim