Played both playtests for Marathon and ARC.
I'll honor the NDA by only talking about ARC.

>ARC has models/textures that don't look like an LOD failure, so that's nice. It also doesn't have weapons that look straight out of an isometric shooter like Synthetik.
>ARC movement is a little slow but not sluggish, and while there are some audio problems you CAN hear people sneaking up on you.
>It also has great PvE.
>ARC's artstyle is neat but a little lacking.
>ARC's 3rd person POV is a drawback, but is comparably negligible.
>ARC's NPC aircraft don't look retarded

Marathon frankly should've been a singleplayer shooter a la Halo and it would've been perfect.