I know you already bought but this may help others. I have one of each with a cab and non-cab
M7040 and then a non-cab MX5100. These are the same as what you are considering. While they look close on paper, they are more than that.
The MX5100 is nimble and handy because of it's shorter size and what I feel is better weight distribution, especially on the front. It grades my long driveway with a landscape rake much better than the
M7040 because of it agility and turning radius. The MX5100 feels firmly planted on the ground because of excellent weight distribution. It feels beefy up front.
The
M7040, especially with a cab, is bouncy up front and doesn't feel like solid weight distribution. It's not that it's bad but if you are driving much above 4 or 5 mph, the front end bobs up and down with a loader. It feels like it "slips" around corner up front. It doesn't feel as stable as the MX5100. In the woods or yard it is harder to maneuver around trees and such. On the plus side, it will lift 2,000 lbs with the bucket but bob around in transport. I love both of these tractors and either one could be considered Kubota's best ever made. Depending on use, I could pick either one.
The
M7040 will pull way more than the MX5100 but as mentioned, is not as nimble for yard or farmyard work. I can't grade the drive with the
M7040 but it will chip 6" wood or load pallets easily where the MX5100 won't. Around the yard, I almost always use the MX5100, even for pulling a rear mower on the lawn when the ground is dry. I couldn't get the
M7040 around trees for that.
The
M7040, for me is for regular ag-type work or lift heavy things, so compared with the MX5100, it gets used maybe 20% of the time. The MX5100 gets used 80% of the time around the house or farm and would be used more if it had a loader. If I could pick one tractor, I'll take the MX5100.
The hydro shuttle is nice on the M70400 but the shuttle shift on the MX5100 is excellent and I kind of prefer it most of the time. I do not have an HST but have in the past and felt the HP loss was more than I wanted for my use. HST's don't pull as much compared with gear transmissions.
On the cab, I love it on the
M7040 but often use the non-cab
M7040 because it's so easy to rip the roof panel off ($1500) with branches. It's easier than you would think. For repetitive field work in the heat or cold, I love the cab but don't get near any trees, branches or buildings. As a result, the edges of fields or yards don't get done with that tractor.
All these reviews are subjective since it's what you want that matters. Either of these tractors would be fine yet neither will be exactly right for everything you want to do. That's why we have light jackets and also heavy coats. If you will be doing home construction and more finesse-type things, smaller is better because you can get closer to edges plus fit around obstacles better. For farm or repetitive larger projects, larger is better because you need extra HP for those things.