I run both. My kids are running the 27T and I am running the 17.5 Yes they are comparable in performance but only for a short while when the 27T is fresh. As others have stated, the maintenance can be a chore, turning armatures, replacing brushes ect.... I am running the Speed Passion 17.5 with the CITIX Clubman ESC. It is fantastic and only cost me $90 plus shipping for each one. This thing will last for several seasons, I can not remember how many motors my kids have smoked. I will be upgrading them very soon.
PS, the only reason I am able to run 27T is because the racers in my club helped me out with many, many spares since they are all racing brushless now.
HPI Blitz ESE (17.5 ROAR)
HPI Blitz RTR