Obtain and carefully study the published specifications of the machines. Plenty of water-lift (vacuum) and sustained air-flow rate are important (a bit like the voltage-current features of a battery . . . if you're electrically minded), along with pump pressure in psi. Also, take the general robustness of the machine seriously, since they take a fair bit of impact from time to time.
Oh, and don't forget the manufacturer's back-up service/maintenance and location etc.
Hope this helps you decide.
Pete (JS2)