I can't comment on how much better the M5140 would be, but I would say it certainly would be "better" for land clearing. The ONLY advantage to the MX5100 is the HST. For a long time, I had a really hard time even thinking about the possibility of giving up my beloved HST.....Now, I'm not so sure it would "kill" me that much.
I think the additional capability of a larger/heavier tractor outweigh the pros of the HST. ESPECIALLY if you've never owned an HST tractor before!
I recently started running my tractor (same size and capability as the MX5100) with the mindset of always thinking to myself, "can I do whatever it is I'm doing, without this HST?" The answer is certainly yes. One thing I didn't like about land clearing with my HST is that you have to have the RPM's up closer to 1800 or so for the transmission. When you do this, your hydraulics become much "faster" than you want them. It becomes more difficult to feather the valves on the FEL. I've done a LOT of land clearing with only my tractor and the implements I have for it, and I can say that if I was in your shoes right now, I would most certainly go with the larger tractor. (actually, I would go with the M7040 if you want to stick with Kubota)
Honestly though, even more important than your tractor decision, is the fact that you already mention getting a grapple. You will do more work with the grapple than you could ever imagine. The ideal combination for land clearing in my opinion, is the FEL grapple with 3rd function valve, and a HEAVY DUTY box blade with top n tilt. (hydraulic scarifiers are nice, but you wont' miss them if you've never used them before)
Remember that a tractor is just the engine that moves all the tools that actually do the work. A tractor with no implements is useless. I didn't have enough money to go out and buy a brand new tractor, and still have left over money to buy implements. So what I did, was buy a used tractor, and purchased my implements used over time. I know have one heck of an arsenal and still have less than 30k dollars wrapped up into my setup. I plan to upgrade my tractor when the time is right, but my smaller, less powerful tractor with all my implements, is more useful than any brand new larger tractor with nothing but a bucket.
The extra weight will help a lot. I had to have the tires filled with 1100lbs of
ballast, and keep a 1000lb box blade on the back, and I wouldn't want any less weight! The problem though, is now that I have all this weight on my tractor, it will strain in anything more than medium range. I continually hear the HST whining like I'm overworking it (I'm not, but it sounds that way) when I have a heavy load in medium. I personally feel that I would get more done with a larger hydro shuttle tractor, than my HST. Although, I do LOVE HST don't get me wrong, but it isn't as important as I always thought it was.
Here is my land clearing setup... I wouldn't hesitate to go with hydro shuttle now, whereas I was "scared" to before. (Dad's running the tractor)
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In this last pic, you can see how thick the trees are in the background, behind the clearing. This is how thick the entire clearing was. I cleared this spot in 3 days of work using this setup. There were a few trees I couldn't push over that required sawing, but a larger tractor would have gotten them