I'd hope it would be preferable both ways.
Personally, I'm a cockpit person, in the 4 years I played MW3, I spent the majority of my time in the cockpit. I was dissapointed with Mech4's cockpits. It looked like more emphasis was placed on 3rd person combat than 1st person perspective.
I'm not designing the interface tho, but I'd hope it would be a more 2D Mech4ish HUD in third person, but actual screens and displays and no real "HUD" in the cockpit unless you had a special eyepiece attached to your neurohelmet or something. In a perfect world, the screens and monitors, sensors and graphs in your cockpit would guide you, and immerse you a bit more as you feel more as if you're really interfacing with the machine, rather than being the machine.
Gee, you guys rock.
Vehicles: I completely agree with you, the hud should be present in the game world, instead of just something seemingly plastered on to your real life screen. For designing the mech interiors, you might want to talk with Tripwire, the guys who made the "Red Orchestra" mod (later upgraded to a full fledged, retail, standalone game) for Unreal 2004. I have not played it, but I read that the realism with which they handled vehicles was quite unique. The gunner couldn't aim with the mouse, and when he looked around he just saw the vehicle interior, so he relied on the driver to aim. Or something like that.
I think, in a 'Mech, the HUD could be sprayed onto the 'Mech's cockpit windshield from a projector at the base of that windshield. In a Tank or a 'Mech in which there is no windshield, the warrior would have to have a helmet that allows a toggleable full 360 degree view or something.
Infantry: Could the infantry HUD be an upgrade that must be purchased (cheaply)? I imagine, before you purchase the HUD, having a system like that of King Kong:
"By default, there’s no HUD (heads-up display), meaning players can’t tell how much health or ammunition they have by looking at numbers on screen. Alternatively, and more realistically, they must rely on more organic practices for this information. For instance, when Jack’s health is low, his vision will be distorted, and his heart will begin to beat. Furthermore, when players reload their gun, Jack will say how many clips he has left."
After you purchase the HUD, it could visually be a projector that sprays the readouts (health, ammo, targeting data?) onto the visor of your helmet, or a thing attached to the corner of your helmet that actually bobs in front of you. Whatever it is, it should render like your gun does, in-world, instead of seemingly pasted to your real-life monitor. Theoretically, the infantry HUD would be connected to a variety of biological health monitors attached to your chest and legs and stuff (like the Crysis Nanosuit or HL2 HEV), as well as to the ammo pack (on the infantryman's back?).
