[Killer Bee]

well, as we all know today, all parts are not created equal.. same applies electric fans.. the 30" oscillating fan in your living room isn't the same as a 30" air mover in the shop..

without seeing the exact model numbers and specs of what you're comparing, I would consider some other factors besides mounting diameter;
elec motor hp
amp draw
cfm output
reversible
bearing life / construction
noise

and for the rest of the installation;
relays
harnesses
mounting style / hardware
shrouds
etc.

several other threads here and elsewhere that have compared specific fan models and assemblies..

remember, reliable heat rejection is the goal so do your homework before buying for best results..

good luck!