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Of course, in my mind, if you have a tractor then there's no reason your lawn wouldn't be flat - the whole point of a tractor is to modify your property to suit your needs, and making your lawn flat would be one of the first things I'd be doing (it was one of the early tasks for my BX2350 when I bought it).
I have a valid reason, or perhaps a really good doctor's excuse. I've only had the tractor since the last week of November 2020. I had rotator cuff surgery on my right shoulder on June 12, 2020, on the left shoulder on October 29, 2020, and I'm just now getting so that it doesn't turn me into a screaming lump to actively (without assistance) raise my left arm over my head. A lotta stuff got put on hold last June after the MRI showed my RH cuff was completely ruptured and the left was getting there fast because I couldn't lift with my right arm. Flat(ter) is a goal, absolutely, and has been for 15 years. Just took a while to save up for the tractor, and justify it to myself. The missus has been encouraging me to get one for a while. I just have to take it easy until I get healed up again, because if I tear my right cuff again, I'm gonna have to have a complete reconstruction. I can think of better things to spend that money on and much more fun things to do.
I may not have been quite clear about the frame sizes. It's the "Big B's" that are going to be phased out, but kind of re-branded as LX models with a few extra bells and whistles. There are some things that were problematic on the "Big B's" that have been resolved on the LX, some of which have been completely resolved on the LX frame. HOWEVER, you are not going to put the backhoe subframe for a Big-B on the LX. Won't fit. Been there, done that, and had it explained to me at length by the local dealer. If you're not planning on getting a backhoe, then that's a non-problem for you. However, it does clearly indicate there is more new to the LX than meets the eye. That caveat is not documented anywhere that I know of on Kubota sites. It is in this forum, though. There are a few things that will be interchangeable with the Big-B's, but by and large, the LX is a different tractor, if only cosmetically, and different enough to be called a new model.
The R-14's on the SU are ENORMOUS compared to the tires available for the 'deluxe' models. I've stood between two LX2610's, one being an SU. It was confusing to see the 'same size' tractor look so much larger. There probably isn't 2 inches between the rear fender and the top of the larger rear tires. One would want to be extremely cautious about putting their hand on the fender while rolling. That's probably why they put grab handles on the fenders.
If you have rough overgrown property (like me), or are planning to move a lot of dirt to level/landscape it (like me), I don't think you're going to be quite as happy with the lighter B model. If you're a lot younger and don't mind some minor increases in work by having to cut things up smaller (like cutting smaller pieces of steak and chewing longer because it's tough), you'll probably be fine with a B. If you want a tractor to run a lawn mower on finished turf and don't consistently need the weight of the LX, then yes, the B or BX might be more in line with your needs.
Here's a good dealer video for the OP to watch, if you haven't already seen it. It's a good run-down of the LX specs and applications. For the price differential, it was a no-brainer to me, especially since the B2650 was nearly $4K higher than the LX SU package while I was shopping. Besides the better price, I'm a firm believer in using a 20 lb sledgehammer for a fly swatter. The OP may have missed the bargain window, though. Can't hurt to ask the dealer and negotiate a good deal.
