It looks like the Halo 3 marines have thigh rigs, similar in appearance ot this:
http://www.airsoftgi.com/product_info.php?products_id=1370
You can mount your pistol holster there if you wish. It's a better spot than trying to draw from your belt anyhow.
As far as the tactical gear goes (i.e. not the armor but little pouches and such), I would go ahead and just wear what is comfortable for you. Just imagine you were going into combat..what would you want, and where for comfort and easy access? What is the Marine you're dressing up as equipped with? What are they packing for?
One guy is decked out with a lot of extra SMG pouches, the other one only has a few mag pouches on his belt. Individual equipment, like in the real world, will vary, and I'm guessing this will continue as the McFarlane toys already indicate Reach army troopers are equipped differently if they're "heavy" or "light"
That's bit of a complicated answer, but the simple answer is that as long as you have the basic armor and the BDUs, the extras may vary.
My advice may not result be a very canonical marine, but I enjoy brining a little real-world military feel type of spin.

Just consider real US marines.
None of these guys are wearing the same tactical gear. The only thing that's the same is their basic protective gear (camouflage, vests, helmet, eye protection, throat protector, gloves), yet they're all recognizeable as Marines due to a few basic elements. I feel it's the same with Halo marine costumes. If you have the right helmet/armor, then I wouldn't sweat the little things like pistols or knives. Consider it all optional.
As far as BDUs go, the Halo3 marines appear to have lighter colored BDUs.