I've got both a 2001 Thundercat, and a 2001 boxeye in relatively good state of health, and they're quite similar apart from:-
The t'cat produces it's power further up the rev range than the fazer, and makes a bit more of it.
The fazer is more tractable around town.
On the twisties, the TC is miles better on the limit or when the bends get bumpy.
They're both really good on fuel. Average 53mpg for the boxeye, 55mpg for the TC.
The TC headlight is slightly better than the boxeye.
TC has slightly better wind protection.
The fazer is comfier (although that's easily remedied on the TC for free)
Every tom, dick and harry try it on with ya on a TC, they tend to leave a fazer alone until you razz it past them
.
I do tend to 'need to thrash' the TC a bit more than the fazer.
My TC is not wearing as well as any of my fazers through the winter!
There's very little in it tbh. Depends on what style of bike your after. Both very capable machines that I'd happily own each of again.