Shorty seems to be most popular. The general consensus is that the car is lighter and therefore more agile and responsive. There also seems to be an issue with motor size if you run saddles; some folks have had to Dremel the steering setup to get their motor to fit. You'll be ok with HPI motors I would think and also believe Orion are ok too.
I've only run shorty and see no reason to change.
HTH
