I want to get into side mount diving and I am looking at purchasing a Nomad. I understand that the XT is modular and if I understand correctly it is made from a more durable material. I will be diving open water only. I am wondering if I really need the XT. The EXP is almost 180 cheaper.
I am not aware that the basic harness and webbing used in the EXP and the XT is made of different material. It was my understanding that the exterior cover on the wing supplied with the XT is supposedly made of more durable fabric, but that was the extent of the difference. If you are diving OW SM - no caves, no tight spaces, no wreck penetration, that is probably not an issue, or necessary feature.
I dont mind getting the XT if its worth the money but I also do want to get the XT if the EXP will work just as well for the type of diving I will be doing.
I echo CamG's comment - having a modular system is generally a better option, even if somewhat more expensive. And, it appears from Dive-aholic's post that you may be able to link up with a shop that can help you go 'modular' at a reasonable cost. One other option that you might consider - you can probably buy, on eBay or Craigslist, a used Transpac harness and a used wing of appropriate lift, then buy a new buttplate and some bungees, and configure a sidemount rig, probably for less than a new XT. If you have access to some experienced SM divers or instructors where you are now, they may be able to help you. You can actually add wing keepers to the wing casing in many cases (if you are careful not to damage the bladder).
Also I will be diving in a drysuit but I am in the Army and if I was to move somewhere new I may want to dive in a wetsuit. Is the dubble bladder worth it? Can I just add it on later if I need it?
For the diving you describe (mostly drysuit, possibly some wetsuit, sidemount), a dual bladder may not be necessary. I do not have a problem sidemounting two AL80s in a wetsuit without a dual bladder, or two HP100s for that matter. I can swim them to the surface. Plus, if absolutely necessary you can ditch your weight belt (if you require added weight in a westuit with two AL80s), and even ditch one tank for that matter. (If you watch some of Steve Bogaerts' videos - gosidemount.com - he dives two AL80s in a wetsuit and only a small hydration bladder for lift.) A dual bladder wing is considerably more expensive. As Dive-aholic mentioned, you cannot just add a bladder to an existing wing. You generally have to purchase a different, dual bladder, wing. One caveat, though - Dive Rite's web site actually says that a single bladder Rec Wing (and a single bladder Classic Wing) can be altered 'later' to create a dual bladder wing. Far be it from me to question the producer, I just wasn't aware that this was possible, or practical.
Will a lift bag suffice if I choose to dive in a wetsuit?
Yes, but I agree with Dive-aholic - it is not ideal to depend on the use of a lift bag as a redundant buoyancy source, without (considerable) prior practice. It is an acquired skill. And, it is a skill well worth practicing. But, if you plan to sidemount double HP 130s (or LP112s) in a wetsuit, for example, you mgiht just want to have a dual bladder wing.
I believe the XT has the capability to adjust the amount of lift it has, can the EXP do the same?
Yes, using the gusset control system. Where others may help you more than I can is with the wing keepers. On the original and the XT, you can slip the wing keepers off the waist strap to gain some additional lift. I am not altogether certain if if you can do that with the EXP - I presume you can but am ready to be corrected if I am wrong.