I find the on ear as comfortable as the over-ears, if not more so. The alcantara pads are super soft, and only get softer with use. Clamping force is a little high out of the box, but that's a super easy fix with a little stretching or simple breaking-in. The pads on the over-ears are often too small for users, something you don't have to worry about with the on-ears. I have no issues wearing them for hours on end, and their small size even makes it easy to lie in bed while listening to some music with them. They're also very easy to wear around the neck and the lighter weight makes them less likely to mess up your hair.
Isolation is subpar on these though. I don't generally want too much isolation because I like being somewhat aware of my surroundings, but you can hardly tell there's any difference when you have them on - isolation is very subtle. That said, once you have some music playing, even at low volumes, it's not that bad. A bigger concern for me is that these seem to leak more than your average closed headphone - not ideal for listening at moderate volumes in quiet places.
As for sound - they captivated me right away, but the bass definitely sounded bloated and slow. It's worth noting that I bought mine used. Like new, but still used, so I don't know how much they'd already been burned in. That said, it still took several days of listening to music to get them to sound their best - there was a noticeable difference in the quality of the bass after extended use. I'd assume this is because of a combination of the usual dynamic driver burn in, as well as breaking in the pad and metal band to achieve a proper level of clamping force. Definitely don't allow first impressions to sway you if you don't like them immediately.
Once broken in the bass is quite wonderful. I find low end response on the original momentums to be a bit boring.This is not for a lack of quantity - the VSONIC GR07 Classic has less bass but is more engaging. It just feels a little soft, and slow. Too gentle. This is not the case with the on ear. The bass hits hard and fast, and extends easily into the sub-bass.
The mids came as a huge surprise to me. I expected them to sound distanced and recessed and bloated by the bass, but didn't find this to be true at all. Listening to the over ear and on ear back and forth with the same tracks, there is hardly a difference between the two. The originals are a tiny bit more forward, but that's it. I actually think I prefer the texture on these too - cellos sound better with the on-ears to me.
The highs are the one area the on-ear noticeably lose to the over ear. Overall treble amounts are very, very similar between the two, but the over ear seems to eek out just a little more detail. Cymbals sound more realistic on the over-ears, for instance.
As for soundstage and presentation, the on-ears excel here too. Soundstage width is just as good as on the over-ears, perhaps even a little better. They lack a little depth and height compared to the over ears, but then compensate for this by having clearer positioning and separation. Instruments blend into one another more on the over ears than on-ears, where I feel I'm able to tell where everything is just a little more easily, particularly in congested tracks. This is made all the more impressive by the fact that the on-ears are, well, on-ears. I suspect this has to do a bit with the sound leakage and weak isolation.
The on ears have often been described as only really being good for bass heavy modern music, but I completely disagree. There isn't a single genre of music I've listened to that sounds bad with the on-ears, whereas I find myself a bit bored with EDM and some hip hop tunes on the originals. I get surprising enjoyment out of classical with these. Perhaps that's because I'm partial to making cellos sound good, something I find these do superbly well. But it's also because of their great instrument separation and texture.
On sensitivity and amping, both of these need a little more power than you'd think from 18ohms headphones to get particularly loud. When I'm on the go, I find myself often having to crank both Momentums to max volume - something worsened by the lack of isolation on the smaller model. As for amping, I thought the larger Momentum would take a clear lead when they were powered up, but I found the opposite to be true - the bass becomes tighter and more textured, and the treble improved to a point where it became difficult to tell if the over-ears were actually performing better than the on ears
One final note: both of these sound better when you use the cable without the microphone. I've never been much of a believer in the power of cables... but it was noticeable here. It's not a huge difference, and it's less pronounced on the over-ear, but notes are more defined, with better instrument separation and overall clarity improvements. That said, the microphones themselves sound clear on a call.
I've spent two weeks with both the on-ear and over-ear, and I still can't decide which one I want to keep. Truth is, I'd probably keep the on-ear for sure if it weren't for the fact the over-ears isolate better and leak less. But on sound alone, there isn't a clear victor. What is clear, however, is that the on-ear are the better value, especially now that they can be purchased refurbished for about $100.