The X739 is gas, but has both 4WD and AWS. If you have a lot of obstacles to mow around the AWS is the cat's meow. And no ROPs is good if you have a lot of small ornamental trees to mow under. The X-series is the best mower and most agile snow mover, whether pushing or blowing snow. And John Deere's engineering: ease of use attachments, etc. is second to none.
The BX's are more tractor-like, and a better value if 3 point implements are heavily used and the mowing situation is more wide open. The BX's give you better loader options too. Though I'm of the opinion few of the subcuts lift enough weight high enough to be an effective loader machine. But, if all you want to do is move around a little mulch and a rare small load of earth or gravel, they are fine for that.
I just bought an X739 that will replace my 425AWS (similar 20 year old JD GT). I love that I can plow snow sideways and still have steering using the rear wheels with the AWS. I've pushed snows as deep as 18" before with no issues on the 425. Expect even better performance with the 4WD X739. I mow about 1.5 acres in about 1.5 hours with many, many obstacles to mow around (30+ trees and shrubs, tight dead end alcoves, flower beds, fence angles, gates, etc). When finished, there is no need for a weed eater because the AWS cuts so close. It's a real time saver for me. May not be much difference if cutting mostly open spaces. Unfortunately, you cannot get both 4WD and AWS in diesel (unless buying a used X749) anymore.
If you use a lot of 3 point implements, or a loader, the BX is the better choice. If you have tight quarters where mowing, snow removal, lawn cart use are your predominant chores, then the X-series is the way to go.
And a diesel is not a must. The Kawasaki gassers in the JD's are very durable and more than enough power. I also like that you don't have to wait as long to warm up a gasser before use (JD claims you don't have to wait at all). A cold diesel can take 10 minutes or more before it is warm enough to apply heavy loading safely. If there are a lot of short restarts, I think the gasser is better suited to this. The gasser is also much quieter at idle, though little difference at full rpms.