I'm not that familiar with the NAUI standards, but from memory (hopefully someone can clarify) there are some significant differences as to how the term DM is used in each. In PADI, a DM is both a teaching assistant and a dive guide, whereas in NAUI I think you only assist in courses as an AI?
Walter is giving some sound advice - if you are planning to instruct, choose on the basis of which agency gives you what you are looking for in terms of teaching. Personally, I feel that there is very little difference between the two agencies - PADI courses aren't as rigid as many people think; but it does take time to develop the experience to be able to work the system to your advantage.
If your plan is to "open a shop" (crazy fool!!), what actually do you mean by that? Is that a retail facility selling dive gear as well as training, or is it going to be a training only facility?
You have to ask yourself which agency is going to (1) give you the most support, (2) enable you to price courses appropriately and (3) give you the recognition. I genuinely don't know which agency will suit you the best - the fact that there is no NAUI facility in your area may work to your advantage in the sense that you may get a smaller number of motivated students, as opposed to walk in customers "wanting to do their PADI". Part of the decision, I think, is being clear about what your target demographic is as an instructor.
Saying that, if there is no NAUI facility in your area you have to be quite pragmattic about it - how would you do a DM programme? I'm sure there is an option in NAUI standards, just like PADI, that will let you complete it 'fast track' in two weeks. But it's really easy to spot DMs who have done things in this way - our DM programme is to a certain extent self paced, but typically lasts between three and six months. You'd have to speak to the nearest NAUI instructor to you and work out exactly what committment you'd be expected to make.
It's not impossible to crossover from one agency to another - I can't comment on PADI DM would fare in a NAUI ITC, but going the other way there are definitely a few things that you could potentially struggle with. The PADI IDC has no diving skills or knowledge components at all - it is assumed that you learnt everthing as a DM... so things like demonstrating skills, knowing PADI tables etc would be needed. A bright spark who is motivated could probably pick everything up in a few days, I'm sure.