I'm going to vote for the EVO based on the fact that you plan on keeping it at least a decade. When I bought my '70 Triumph TR6 in '75 there were shops everywhere and plenty of parts. By the '80s we went through a shortage of spares and I thought that would be the end of keeping my Triumph. It all turned around by the '90s and every part is available and it potters along just fine. It's the parking lot here at work.
That's the case now with the EVO. You will never be in need of a part for that bike. I'm not sure about the '02 TC since HD is still trying to sort out the cams and balance system on that motor. There are several iterations of the twin cam in a short few years that wasn't so much the case with the EVOs.
But, if you pass up the Twin Cam you will miss the balanced motor and more stock power.
As for uprating the EVO. What's not available? The motor is solid and well engineered. At 83K miles I'm still on the original belt, clutch, bottom end, tranny etc. I did a top end rebuild at 75K, but only because I had the motor apart to replace one lifter.
One last comment. The '97 EVO is 10 years old now and if you were here in the States the HD dealer will be blowing you off, or gouging you for every part. With the Twin Cam you won't need to see the parts dealer and they still have whatever you need in stock.
Well, here is one thing to consider. The '97 is very close to the cut off for service at the H-D dealer. Some already refuse to work on Evo's as they are too old. So unless you are going to work on it yourself or have a trusted mechanic, go with the Twin Cam. Personally, I own one of each...the Evo hasn't left me on the side of the road yet (neither has the TC).