I have the MX 5000, I think it is a great tractor for our needs. I do a lot of field cutting, gravel road maintenance, pasture maintenance (harrowing and manure removal). We have also used the tractor to run a 25kw PTO generator for backup power to the house. I have also used the tractor to put in hundreds of fence posts with a post-pounder attachment. Some of the tasks we do involve loader work, but most of the regular tasks involve picking a gear and staying in that gear for hours at a time. This was the main reason I felt the gear tractor, with the most HP I could afford would be the best choice for me.
One other thing to consider is the number of different people using the tractor. With my tractor, it's pretty much just me. If your fiance will have employees driving the tractor, the ease of use with the hydro may be a benefit. Remember that the MX5000 (not sure if it is the same with the 5100) has a partially synchronized transmission. 1st and second gear (low or high) are not synchronized and you are supposed to come to a full stop when shifting from first to second gear. 3rd and 4th gear are synchronized so you can shift on the go from 2nd to 3rd and 4th. Your fiance should really drive the MX5100. If it were me, and the jobs were in tight spaces, I would get real tired, real fast with all the clutching, shifting and stopping. The difference in HP between the two tractors is not that great and may not be a factor depending on the specific job and/or implement you are using.
As far as ruggedness and reliability, I could not be happier with the MX5000. The tractor has had zero issues and puts up with a lot of abuse (from the conditions and the operator). I have done all the required maintenance myself which has been pretty straight forward (and believe me, I'm no mechanic).
I don't have any experience with the L4400HST but like I said, he really needs to drive the MX5100 and consider the kind of work he will doing everyday.
Hope this helps,
Jeff