Stephen – every 6 months or so, I look at this mill because I think that it would make a really nice little "CNC Chucker" lathe.
There is an older review of the U1 at **LINK**
From looking at the Sieg website this past weekend, the differences between the U1 and SU1 are in the motor, as Ketan suggests.
I'm actually quite familiar with a Centec 2B, with quill-vertical head, and I don't think that there would be any comparison, but I might be surprised. (smile) Over here on my side of the Atlantic, the Grizzly G0727 is possibly available in a show room about 10 hours drive away; a bit far to go to twiddle dials; and it'd be in inch anyway. (not an issue with CNC, of course)
I also seem to remember someone building a column extension block – sorry, don't know where I saw it, but google the Grizzly name/number, and you never know what may pop up.