Its obvious that you should go for the RE0 first as they provide 30day refund
Isolation:
Subjective.
I have poor isolation with RE0 and slightly better with denons.
I think its due to the small air vent holes in the earphone to compensate pressure.
Build:
RE0 cable length and nylon texture is better. (denon wire is too freaking short n cheap rubberly texture)
Denon have better wire housing quality. (earphone casing and wire jack casing may split.)
RE0 wire is thicker and denon are very thin, hence you need more care for your denons.. (they give u a nice small leather case that serves its purpose though)
Knurling on the RE0 is subjective.. it may hurt your ear.
Sound Quality:
Mids of denon c710 and re0 are around the same.
Denons have better lows
RE0 have better highs
Denon bass is alot better
depends on your ears and what u like.. (will be great if denon or re0 have best of both worlds)
After Amp (if u intend to get one):
Denons are around the same.. (might have louder hiss)
RE0 benefit through bass boost but the bass is still not as good as denon.
Depends on what amp u use actually.
I will recommend an amp to go with the RE0 even if ur player is a cowon d2 as i tested it with my d2. (with n without amp - bass boost is very significant). Never go with Ibasso for REO and D2 as it will distort alot. E5 fiio works well.
Still.. this depends on your liking and your eq settings.
hope this helps