Thanks for the info.
Pretty sure i need the 52 because we have about 2-3 acres planted with trees and gardens planted close together. A larger machine would not fit in some of these spaces. Too bad, because I would have liked to get the larger mower.
Upon reflection, the vibration may have been because my fields are too bumpy to run at top speed. From the reading I have done since, I am pretty sure this is the case.