Well with the JD you are getting alot more for that $250 extra than just the BBC as you pointed out. Look close at them now though. In the past 2 years the JX75 has really gone down hill bad in durabilty and quality of machine. IMO, look at other brands too. Honda commercial, Toro proline, Exmark. Even some others like the Ariens commercial I think are better built than the new JD's. Plus JD is notiorus for having weak and expensive transmissions to replace and the wheels being all plastic wear really quick.
I have an older, much older, JD 21", and I love it! I don't use mine very hard at all and it had the BBC replaced when I bought it because it was seized. This last summer I spent the $90 on 4 new wheels only to have the transmission shell out 2 weeks later! That was another $180. I went ahead and spent the $$ on it because it's a good solid machine and the engine starts on the first pull all the time. Plus it's NOT replacable anymore...