Funny you should ask. I use a TS55 for cutting 3/4" plywood on a frequent basis (much easier to use than my Powermatic 66), and rarely use my TS75. But, just yesterday I had a need to cut about 1-1/2" off the legs of a router cabinet, and the 55 did not have the depth of cut necessary. I simply attached the guide rail across two legs and presto, a nice square cut that was easy to make. I recall the same procedure several years ago (pre Festool) when I used a traditional circular saw and a guide board. The difference in both time required and the quality of the result is amazing.
For now, get the 55, but don't be surprised if you find yourself with a 75 too.
Jim Ray