The technical terms are supra-aural (on-ear) and circum-aural (around the ears). Both can be sound-wise excellent, but circum-aural is definitely more comfortable.

I used to own a pair of Joe Grado HP-1 HP-1000 supra-aural headphones and they were excellent and did a great job with binaural recordings. Since selling those, I moved on to the Denon D7000 circum-aural headphones. Far more comfortable. Bar none the best audio experience I have had was with AKG K-1000s hooked up to 75-watt tubes monoblocks. However, that wouldn't have worked for VR presence because of the latency from the tubes. With solid state speaker amps to keep the audio in sync, the presence would probably be unreal, assuming the K-1000s do well with binaural recordings, but that is something I'm uncertain of.

I'm not arguing about sound quality though. When I said "immersion" I was thinking about over-ear headphones generally having better isolation than on-ear.

Sounds quality isn't really dependent on circumaural vs supra-aural headphones. The major complaint I hear with supra-aural headphones is that they apply pressure on the ears, which can be unconfortable. Sound quality, however, is largely affected by the drivers.

Will these apply much pressure on the ears? Many headphones use the earpieces to stay in place, but these are positioned by the Rift's straps which should be able to do it without pressure from the earpieces.

This is correct. It's called listening fatigue.

It's not just listening fatigue from sound, but the pressure applied to the ear-lobes. I have used circum-aural headphones that I can use for hours upon hours since they were very comfortable (no pressure) and were not too bright or forward that they caused listening fatigue.

Yes, I have a pair of Grado S60 (supra-aural) which are by far my best sounding headphones, but I cannot use them for many hours on end for this reason, as loose as I try to set them, they eventually become uncomfortable.

Its about the noise canceling.