I would say the Master Chief would be the hardest especially if you use the High Def Pep files. The ODST would be more expensive i believe as there looks to be more individual pieces that would require resin/bondo, therefore you would use more product.
omg, i already voted, but i mistaked the WH40K Space Marine for the Halo space marine D: the Space Marine wouldn't be to hard to make, but its going to be BIG, lol. you WOULD have to use stilts, and do some modifications for the hands so that you can curl your fingers..... lol...... if there was a vote, the WH40K Space Marine costume at full size would by far be the hardest to wear
I am building the full 40k suit right now. The pep is not too much trouble, but the overall surface area is massive and it lacks the intricate detail of the Halo goodies. To get a reasonable height, the boots are going to be modified with 5" rubber sole slabs attached to my old Army boots. The backpack and pauldron weight is pretty comfy as it's attached to my backpack frame (Army rig). Who knew Uncle Sam could be so helpful?
Cost is about the same for materials, but you if factor in all the extra sanding on the 40k as well as the lack of "ooh" factor, I say go with Halo.
Highest cost would be the Chief just because of the details and curves it has, and because the ODST requires less bondo, and the 40k suit can be mostly scratch built. The harder one would probably be the Chief, too.
I'd go with a Halo costume, though, if for no other reason than the fact that 100 times more people will recognize you.