First off, no HST+ on these models because it is a B-series. None of the other B's have it yet and probably won't in the foreseeable future due to the cost, complexity and size requirements of that system. Since the L45 is built on the Grand L platform it was natural they included it in that tractor.
As for the comparison between the B26 and B7800, having used both extensively (well, a B3200 and B26) here's my take:
The B26 is great around the job site and doing other utility related tasks. The loader is impressive for a B-series (in fact I think it out specs the L3200/3800) and the hoe is also very nice. You truly get a lot of earth moving performance in a small package. Of course, the skid steer quick attach is also nice along with all the hydraulic options, position control 3pt hitch, etc.
The B7800/3200 is much better in the power department. Whenever I use a B26 the power difference is quite apparent. Things that you wouldn't think of as a problem with your B7800 really slow down the B26 (like going up a steep hill in Medium range with a full bucket). PTO work is also a noticeable difference. My B3200 handles a 5' bush hog in most every condition with ease, IMO 4' would be better for a B26, but then it doesn't cover your tracks. Another thing is the B26 feels a good bit bigger, especially in the woods. With the wide loader frame, fixed canopy, and large hoe hanging off the back it can be difficult to maneuver in the same spots I take my B3200.
For me the extra power of the B3200 and removable loader is more beneficial than the extra loader and BH capabilities. I use mine for as much farm related work as I do construction and for that the B3200 is a better option. If I were using it for 90% construction, landscaping, trenching, etc work I'd definitely spend the extra $$$ for the B26. The B26 is a very nice package, and I sure wouldn't mind having one. Aside from the above mentioned reasons, the other reason I went with a B3200 was of course cost, with the B26 being about $7500 more (both similarly equipped)