I had an NX80V and have a TH55 now... so these are my opinions.
Screen brightness - The NX is too bright in a dark room, even on the lowest setting. I think the TH's more than bright enough, even for outdoor use. Launcher - Don't think you can do anything to the NX launcher, you'll need a third-party launcher for tabs. Free notes - I think earlier Clies have an application for that, but you can't hyperlink. Software in general - I can't give concrete examples, but Sony says they've done a lot of software optimization during the last year, and it shows on the TH55. It feels faster than my UX50 and the UX50 felt faster than my NX80V for some tasks (NetFront, Picsel Viewer, the system in general). Remember the Clie Organizer MemoPad lets you import .txt files larger than 4K. PIM apps - the NX80V come with standard palmSource apps. There may be third party alternatives for hyperlink and such, but for PIM purposes the TH55 is a far better choice.
Sturdiness - The NX80V is very solid except for the rotating hinge of the screen. My NX80V dropped off the dining table, and the screen has been misaligned with the bottom ever since. Remember Japan has monsoon season too, so the memory stick slot door thing shouldn't pose a problem.
Tablet PC-style handwriting input - I don't think there's anything on Palm OS that does this (writing on the screen and it converts text at the same position on screen). You'll always be writing at the bottom of the screen, and text is entered in the display area above it. You can customize handwriting recognition on "Decuma Latin" (not the Decuma with Japanese market models), that'll improve recognition.
Battery life - The NX80V's got a huge battery, but I think the TH55 still has a bit more battery life.
My last opinion - I bought the NX80V thinking it'll replace everything (mp3 player, digital camera...) but I was dissatisfied with these capabilities - they don't perform as well as individual devices. The TH55 is focused on PIM but does a wonderful job at it. Also, I think so much software are improved and perform better on the TH. Happy shopping!