I have a TM5 and a TM31 which I've had for 4 years. I love both.
Some of the things that annoy others about the TM5 (like the delays with the lid opening) don't worry me. I love the new technology and the touch screen.
I still love my TM31 and use it often (today I had both going at once).
The 31 is a wonderful machine and if you are happy with it, there really is no reason to upgrade.