I typically make a pumpkin beer every year.
I've used whole pumpkins (cut up and roasted) but that was years ago.
I typically use canned pumpkin - Libby's solid pack pumpkin, or an organic brand depending on what I can find.
A couple years ago I did use the Libby's pumpkin pie mix due to a shopping error. The beer came out great though I can't say it was specifically due to the mix. The canned mix certainly didn't hurt though.
So, my experience is that you can make good beer with either approach. Canned is less work.
I've found that I prefer the spices added in the kettle rather than in the keg. The difference being cooked spices (kettle) to raw spices (vodka tincture). I suppose you could make a spice tea and add it to the keg, which maybe I'll try if I brew a pumpkin ale this year.
FWIW - I had a heck of a time finding canned pumpkin last year. Once Halloween hit it was widely available, but finding it around this time of year (August/September) was tough. I even tried to special order it through the local health food store.