I compared both of these models when I bought. I also briefly considered the L2501 but I ultimately chose the B2650.
The B2601 is the current equivalent to the tractor I had (B2410). For my uses, that size worked well for me for 11 years. I have just shy of 6 acres with a mixture of finished lawn, pasture, and some woods. For most of the time, this was was my primary lawn mower with a 60" belly mower so I appreciated its lighter weight. I used it with a back blade for plowing snow I the winter and planted food plots on my hunting property in the summer. It had plenty of power for pulling a 5' disk, a 5' brush mower, 5' box blade and several smaller implements. The driving force for me to buy a new machine was the FEL. I didn't necessarily need more weight capacity but I would have liked it to lift higher and I really wanted a quick attach of some sort.
For the most part, I was sold on the B2601 as a replacement. It has the better loader features I wanted and I felt everything else was comparable to mine.
When I sold mine, I began my search in earnest. When I got quotes on both B2601 and the B2650 they came in $1100.00 apart. I'm pretty frugal (cheap) but that difference was close enough for me to really consider the B2650. It has a better loader (more weight and higher height yet) and has better performance specs all the way around so that became my choice. Early on, the dealer offered me a L2501 for the same price as the B2650 but I was concerned it would be underpowered. It also doesn't have some of creature comforts of mine (seat, loader control is more ergonomic, mid pto) plus I didn't like the way it fit me.
I've had this new tractor for only a month or so but here are some observations. First is it is pretty light, especially in the rear end. I almost tipped it the day it was delivered. The first thing I did then was to load the rear tires with RimGuard which helps quite a bit. I might even add some spacers to the rear too but we'll see on that. Second, this machine is quite a bit more powerful than my old one. We've had one small snowfall which I plowed without the slightest hint of slipping or the rear kicking out so I'm pretty pleased with the power and that extra weight in the tires. Third, this engine and HST are both quiter than my old one. Fourth, I too think the sheet metal flexes a little too much (at least compared to my old one). I read somewhere that the sheet metal is mounted on rubber so it's not really flexing but rather moving because of the rubber. This is done to help with vibration and noise. Fifth, make sure you get the SSQA. It is still an option on the B series and some of the dealers that put packages together use the pin on style to keep the price down.
All in all, I think I made a good choice for my needs. I don't need to mow with this tractor but I will still be using it on my finished lawn so I don't want too heavy of a machine. It should do all the things my previous machine did and probably a bit better.
Good luck with your decision.
John