Do you enjoy reading the manual and have a "journeyman" attitude towards your machinery in general? Go Summa. Happy with a cut that's 90% good enough and an irrational fear of cut strips? Go Graphtec. ;p
My personal take from reading every single post on the two from everywhere on the web for the last 5 years, is that Graphtecs are slightly easier to just get going, and Summas are slightly better at eeking out that extra 5-10% speed/accuracy/quality, if it's important. My Summas have been bombproof to the tune of about 2 million die-cut stickers over the last few years, with the main machine still being the used 2015 S2T I got on Craigslist. Unless Graphtec came out with a whiz-bang tangential model, I've got zero desire to ever consider them, even at half the price.
A lot of the comparison issues could be as simple as
Drag vs Tangential?
If you're not coming up with a quick gut feeling towards either one, flip a coin. There's a LOT of folks happy with each one. Both seem to be well built and rarely break down without abuse, and are supported by major RIPs.