UTM: how to measure speed, throughput and efficiency in firewalls?

I´m on a project to test several different solutions ofr UTMs and the future
version of ISA Server (TMG) will be tested too, as soon it will be available.

Now it´s scheduled to test some hardware appliances (WatchGuard X/Peak
Series, Fortinet 310B, SonicaWall NSA 5500, IBM ISS MX, SecureComputing
SideWinder and Kerio and TMG)

Now, i have 300 users, 15 Servers published in the Internet, 24-day
operations, 5 locations across the country and i wann enabel ALL security
features available on UTMs, like AV, AS, IDS/IPS, Web Filtering, URL
Filtering, authentication, Proxy-Cache, reporting capabilities, etc.


I´m cunfused by the manufacture´s technical details and i don´t know how to
really measure the efficiency of TGM comparing to other solutions.

Concorrent connections?
throughput?
SSL VPN throughput?
IDS throughput?
AV throughput?

Some products achieve 400.000 concurrent conenctions, others 600.000 ou
900.000 simultaneous connections

Some products achieve 500 Mb/s of firewall throughput, 100 Mb/s of VPN
throughput, sometimes 10 times this values.

How can i ensure i´ll choose the best solution, to achive the performance
level the company needs?