The only thing that the standard U1 has here is a DAC and analog output. If you don’t need this and are still intending to do the Sbooster mod on the U1 MIni, then I would say go for the U1 Mini. The digital transport section is the same on both.The U1 does not have a DAC, it is a digital transport like the U1 Mini. The "only" difference is that it is a much better transport.
That being said, I took a close look. I think the U1 Mini + Sbooster is probably going to be equal to the standard U1 if you just use them as a digital transport.I used to have the U1 Mini and added a good LPS (Kenneth Lau Signature). I thought I had a transport as good as the U1 for half of the price. Well I was naive and wrong. I took home the X1 for a test and connected it as a transport (same like an U1), with the same cables and Aesthetix DAC I used to have with the U1 Mini.
The difference was huge, the weakest link in my system was the U1 Mini with LPS and not the DAC what I thougt. There is a reason why the U1 costs almost 3 times more than the stock Mini....