How are you planning on carrying tools, food and extra layers? Rack w/ seatpost clamp? Carradice? Handlebar bag? Camelbak?
I've done the Boston and Westfield brevets on a Trek 520 (analog to your Trek 500) and while I wasn't completing them quickly, I was still finishing, and was able to cap off finishing a 600k.
To khearn's point, we do get rain on a relatively frequent basis in New England. It's certainly not as omnipresent as the Pacific Northwest, but after three years of randonneuring here, I am hard pressed to identify a single season that was completely dry. Last year's Boston 600 alone, was conducted in the midst of a zero-visibility lightning storm. So, I think fenders are definitely worth considering.
Roads in New Hampshire are fairly rough, and if you're going to be doing the Boston series, then you will need a bike that can deal with chipseal and potholes (less of a concern if you're going to be riding more of the Westfield rides). The Scattante's aluminum frame has me a little suspect, but wider tires run at lower pressure might offset that.
We also do some seriously hilly rides, with the Boston 300 and 600 being ones that primarily come to mind, but even the 200 has a lot of climbing. The Trek 500's weight might be a bit of penalty there, but the extremely wider range of touring gears came in very handy on the second half of a 600, when it came to spinning my way up the Green Mountains.
I would say that you ought to give the Scattante a first go. If you can get through New Hampshire without any serious trouble or soreness, take it on to the 300. If you get beaten up, then bring out the Trek. Main caveat is that regardless of your bike, sort out how you're going to bring your gear.
I would caution against using a frame bag if you have ITB issues, as I had a friend who found his ITB issues excacerbated with a frame bag, as the bulge in the frame forced him to move his knees outward. I am also not a big fan of wearing Camelbaks throughout a long ride as I find that the weight just hurts my back and shoulders over time.