Hi,
What is the expected throughput and concurrent number of connections? Will there be databases behind these servers, do you need two-tier firewall for this? Do you need Active/Active, HA, or just SA firewall? What is the network connections, will the number and type of interface for ASA 5510 is enough? How about future expansion, what will be needed in the near future? Thus the vendor partner in the city I'm located have a good support for this firewall? These are the basic questions needed to identify the right firewall.
Check this link for model comparison http://www.cisco.com/en/US/products/ps6120/prod_models_comparison.html
Regards,
Dandy