I think the bench is only available in the Tradesman or SLT. What I found last spring was that most Tradesman trucks on lots were optioned with all the base SLT features, so they aren't really stripper trucks - keyless, auto headlights, power windows/locks, chrome grill, bluetooth, USB ports and etc. A stripped down Tradesman is probably equivalent to an XL Super Duty, but the more common well optioned versions are basically equivalent to an SLT Ram or XLT Super Duty.
There is no difference in engines or anything functional based on the trim level. It's just a matter of how many features you want.