Thanks for all the help. Now let me ask this... the dive shop in question offers the cert for $200. How is that price-wise?
That's expensive IMO. Nitrox use is so easy and simple that it should be a low online course fee, that's it. Try to get a nitrox certification bundled with some other training for a much-reduced price. It's not that hard to do, and since there's almost no skill involved, the teacher is not nearly as important as with a more involved skill class.
If the shop told you that nitrox increases bottom time AND increases safety, maybe they need to take the class again. It can increase bottom time, and theoretically it can reduce DCS risk, but it does not do both. I bet someone has already brought this up in this thread.
It's very important to understand that nitrox has not been shown to decrease DCS risk in recreational diving. Making incorrect assumptions about safety is in itself an added risk, as it is likely to affect dive behavior at some point.
Regarding the fatigue issues, I have found that extending safety stops and practicing very slow ascents from the stop also reduce fatigue, particularly on multi-dive days. I believe the fatigue (this is a theory, not a fact) is mostly due to residual N2 loading, so lowering your N2 through any means 'should' have the same effect.
Nitrox is very useful for liveaboards and other multi-dive trips, as it does really increase your ability to do many dives to substantial depths in a given day and stay below NDL. For a two-dive day it probably won't affect your dives much, especially if you're in a group with both air and nitrox divers diving together with the same profile.