I have found both UTMS (the original version) and FV to be very effective. For my personality and style of working, UTMS is a better fit and has proven to be the best of Mark's systems for me.
I think that UTMSI adds too much resistance without significantly improving UTMS.
I'm going to correct myself that it's probably too early to say UTMS is better than FV for me. It has some clear advantages: I like having one task selected at a time, and it gets unfinished things finished. However, FV has a lot of advantages and has worked well for me for a long time, so I think it's too early to say which is better overall.
UTMSI is working well except my ridiculously long "new" list. It has been awhile since I used FV, it worked too but in a different way. Best thing about UTMS, either one, is things get finished, even if ever so slowly.
I think that UTMSI adds too much resistance without significantly improving UTMS.