Both are great cars. The SPF comes standard with Top, Tonneau, heater, defroster,
oil cooler. In my opinion, I equate the two like this, the BDR represents a 1500 series pickup turck the SPF represents a 3500 series pickup truck. The SPF is an extremely solid replica, you can hit railroad tracks, chuck holes and never experience a panel rub or squeak. It comes with the authetic mechanical smith gauges up to including the Lucas amp meter. It also has a very nice wooden steering wheel. I also like the belcwat? windshield frame as the originals had one. It does have the shelby licensing (let the flames begin). Traditional 90" wheelbase. The components (suspension) on the SPF were designed for the SPF MK III replica. Other replicas design their cars to accept components from other vehicles, ie, BDR utilize BMW components.
The BDR is in my opinion one of the best buys for the buck, they have come along way and come standard with stainless steel sidepipes. You could not build your own replica for the same money as a BDR and get the same results, the BDR will be better.
You can find alot of BDR's between 35K-45K up to new in some instances. You can find alot of SPF in the 40K to 55K, for a new SPF you will be in the mid 50's. The difference in a new SPF and new BDR is probably $8K which you will get back on resale and in todays market may make the difference between selling your cobra and not.
If possilbe view them both on a lift and you can see additional differences.