Which is the best value for the money depends on what you want to be able to do with it.
If you don't need the extra capability of the PFC3BXL and the additional 512MB of memory (for 1GB total memory), the Supervisor 720 with PFC3B and only 512MB of memory should be fine.
I have a pair of 6509s with a Sup720-3B in each, receiving full BGP Internet routes and switching 34 Gigabit Ethernet connections, with ingress and egress ACLs on 28 of those 34 Gig Ethernet links. Memory utilization on the Sup720 is at 45%, and CPU utilization averages 5-10%. Plenty of room for growth, in my case at least.
You can save some money now by buying the Sup720-3B now vs. purchasing the Sup720-3BXL. If you find you need the capabilities of the -3BXL later, you can purchase the upgrade kit. Cost of the kit, at least at list price, is equal to the difference in cost between the two Sup720 modules. But you'll need some downtime to perform the upgrade.
If the cost of the downtime for the upgrade will set you back more than the cost of just buying the Sup720-3BXL, then go with the -3BXL up front. Saves you the hassle of upgrading later. (I was going to buy the -3BXL up front for exactly this reason; but downgraded from -3BXL to just -3B to free up budget money for another project.)
