Ally are ok for working on a perfectly smooth surface, if your working out in the street, id be wanting a steel frame.
I have an Arcan 3ton from Costco, was on special for 75£ about 4-5 years ago, tis a beast (a heavy beast, but still a beast!!)
http://www.amazon.com/Arcan-Quick-Li.../dp/B000Y9YGX4