I have the Tonic, because it was quite a bit cheaper. But if I got one now, I'd get the SU since it has much clearer measurement markings of white on black and black on grey. The Tonic has indentations in orange and orange printing on grey. Check out pics of both and it will make more sense. If that isn't a problem and your vision is better than mine then the Tonic is a better buy.
Cutting wise, both are the same. Works great and I can shave off slivers of cs and cut poster board and chipboard. It's self sharpening and the blade isn't scary sharp like old fashioned guillotine cutters. Only thing you can't do is cut out a square or rectangle in the middle of cs like you can with the Fiskars type trimmers.
Cindy in Oregon