I have seen many a post asking for vacuum preferences and it seems that vacuums are like cars, everyone has a favorite for completely different reasons.
I would suggest these thoughts though when you are comparing vacuums for any type of professional cleaning.
1) suction that bypasses the motor to keep your fans from getting damaged when solid items are sucked into the system
2) a long commerical cord (as you previously stated) so that you do not have to constantly have to find a new plug every 20 feet.
3) ergonomic handles to aid the operator from having hand and wrist problems
4) a manageable weight for carrying/ lifting (consider your smallest employee as well as your average employee in this decision)
5) on board attachments for above ground cleaning with a hose that is extendable a good distance from the vacuum without tipping the vacuum over (if you are looking for an upright as compared to a backpack vac)
6) a decent warranty on parts and construction (minimum 1 year under commercial usage)
7) decent pricing on replacement parts as well as any bags, belts, filters, etc... This is where some manufacturers will stiff you badly
FYI - We have a great vacuum vendor who will gladly discuss your needs with you and provide to you several options to choose from in our purchasing alliance. If you would like their information please let me know and I will gladly pass it on to you.