Performance - Only buy Aerzen's blower packages... just kidding, but there is a serious point here. Any manufacturer's claims of performance need to be verified to insure the correct amount of pressure will be generated for the application. Insist on knowing the performance and efficiencies (power requirements) of the complete blower package you buy not just the performance of the bare stage. An undersized drive motor will limit performance.
Package design - Buy a complete package from the manufacturer when possible. A package assembled by others can consist of mismatched components that could have a negative impact on the blower's performance, and ultimately, its warranty.
Pricing - I put this last, because it is the least important component in the blower selection process. A decision based solely on a lower up-front capital cost can end up costing much more in the long run when cost of ownership is evaluated and the inappropriate, and possibly, less reliable equipment was selected.