If the site already has 50 users, and there is expectations that more/all users may be allowed connectivity in the future you should be careful about using the Pix 501. The 50 user license is a MAX. You cannot license it for more connections. The hardware is more than capable of supporting it, but Cisco has done this apparently to make the Pix 506 more attractive.
So if you have growth expectations over 50 users trying to access the outside, then you should get the 506 instead.