I own the T5 61, I really like it, because I can use the case that I used for the T4, T3.....a friend of mine got the T5 76 keys and he needed to get a new case, it is HUGE.......and he after almost 3 years with it, likes the range (keys) but the size to load and unload is getting to him.... of course, also the extra weight.
Both are identical internally... the difference is if you install the optional memory and premium packs and that makes a big difference.
Sonically and feature wise, can not go wrong with either......
Both have the same key-bed action. so, it will amount if you are willing to get a very big case to protect it, is almost silly to get a T5 and a soft case that will NOT protect it.
In the other hand, and most importantly, if your playing style require more keys, the 76 is your board.
Just my 2 cents....
Manuel
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mdorantes