First, because I often like to use my headphones with higher impedance devices like my old integrated amps or studio/recording gear, then second because It's a little cheaper and the ear pads look more comfortable. I don't think I would use headphones like this for anything portable.
I think the gmp 8.35D gets much better when hooked to a more powerful amp. It sounds much better on the fiio e9 than on the fiio e7 for instance. Soundstage increases, overal detail and prat!
I think you can't go wrong with the 8.35, but please do buy the oval earpads with them.