Two sups will give you supervisor redundancy, but not physical chassis redundancy.
That really would depend upon your needs for uptime. Ideally dual chassis is better in case something catastrophic happens such as a power supply bursting into flames. Not hugely likely, but still possible.
The choice of the 4507 or 6500 series would depend on several things.
First off, which sup module are you looking to go with? The Sup 5 10GE on the 4500 series doesn't currently support full OSPF functionality per my last discussion w/ Cisco a couple of weeks ago.
Secondly the 6500 series would allow you to scale to 10GE closet links for access in the future should you need to. There are no 10GE line cards for the 4500s at this time.
6500's allow traffic bound between two interfaces on the same card to pass without hitting the SUP. 4500's require everything to pass through the SUP.
Then there is just plain throughput difference. There are 1.1 to 1 Port to ASIC cards available so that you have a lower over subscription rate.
The 6500s also allow you to use integrated service modules such as IDS or an FWSM, or even wan modules.
There is an obvious price difference, but the 6500 series seems to give more growth, raw throughput, and flexibility.
