Only a $250 difference on an 8' box? Is that a light duty box or a very light duty box?
The swinging rear blade allows the front blade to dig in with less restriction (in swing mode) with nose down) so it digs deeper, faster and easier. When bolted down, the rear blade can be used tilted nose up, to smooth the ground without any digging action from the front blade.
I looked at your profile. I hope you are looking at boxes like the Gannon Industrial series like the high backed landscapers or similar. Either with or without hydraulic scarifier bars. Considering the power of your tractor, the hard Texas clay etc, you had best be getting one that has a weight in the 1500-2000 pound range. If not, you will bend it up fast.
jb