150k on the TDI is just getting broken in. Plenty of 300k+ TDIs out there.
The biggest question is how much driving do you do? I love our TDI and our old VW diesel, but the premium you pay for the TDI, you will only really make it up if you drive a lot. I have a 13 mile round trip city commute to work, so for me, I wouldn't really make up the cost difference. My husband drives the TDI to work and its our trip car so we put more miles on it. We also got a heck of a deal on it
TDIs do sell for over market value. That price may be a tiny bit high, but its certainly not an out of the ordinary price for the car. Finding a TDI with less than 150k at that age is difficult. Depends on the maintenance history and stuff too though...timing belts are critical, correct oil must be used, etc.
GLIs are nice though. I have a 20th GTI and love every minute of driving it. Its faster than the TDI of course, and the 6-speed is awesome.