HMR are more durable; so far only Jenius' shoulder broke. Yammy have had many shoulder breaks on the early V2 1/60. See Jenius webpage for the list of broken valks.
The plastics and the large (non moving) parts on the Yammies seems higher quality, but the transforming hinges, parts, etc doesn't seem much better than Bandai HMR. If the parts break, you'd have to get them printed. yammies look really good on display because they are more detailed and larger. Like models, fragile but detailed. Also expensive. If you have the cash, this is the best valk currently on the market. Yammies had a couple destroids.
Bandai HMR has a softer plastic that feels cheaper. But the softer plastic doesn't break as easily and isn't brittle. I've popped out the shoulders (they can pop out) and put them back on. HMR is simpler to transform and smaller and takes less space. They are like toys. I don't worry about breakage. Also HMR has the enemy mechs, and more destroids so this line has more to collect.
This. The partsforming doesn't bother me; it's just the cockpit windshield/heat shield. The landing gear is hardly used by me.
The 1/48 Yammies are really loose by now, the V2 60 were not as loose because they tabbed into the other parts (but some V2 were prone to breakage). The Bandai copied the V2 tabs. The Bandai HMR have been getting tigher in the recent releases. My HM (not HMR) from years ago has gotten looser but still pretty poseable.