Well in the book it says it's a B. But you could probably look it up online or something, because the book we have at our gym is old so it might have changed. But I know it's not worth more then a B and if it were a C you wouldn't be able to do it in your routine anyways.
FWO and BWO are both valued as the same (or as my coach continues to tell me) atleast in USAG they are. From my knowledge of the sport both are "A"s, i remeber checking the Code a bit back to make sure and they where as such. In AGA a FWO is a "B", but that is irrelevent to USAG.
That being said, i perfer FWO over BWO because i have done them since lv 5 while BWO have always given me trouble. They are a lot harder on your back if you are not extremely flexible, after taking a break of about a month or two and coming back to them i was completely unafraid but after around 7 turns i was again reminded of my back pain that i had forgotten. I love FWO and do believe they are "worth it" even as an "A".