Hi,
Nitrox has a few benefits, including not being as fatigued after a few dives in the same day. It will also let you stay underwater longer. The only down side to Nitrox in my opinion is air fills/rents tend to cost a few bucks more.
It seems strange that your local shops are offering both these classes combined. But, I know why they are combining these classes. Per PADI training standards Nitrox dives are the only specialties dives that may be combine with other training dives.
Let me give you an example:
Let's say you go on a dive with a PADI instructor at night to see a ship wreck. The instructor can NOT give you training credit for a night dive and a wreck dive. He can only give you credit for one or the other.
However, if you made a night dive while breathing Nitrox the instructor could give you training credit for both the night and Nitrox dive.
In your PADI open water course you are required to make four open water dives. If you later you decide to take the Nitrox course you are required to make two additional dives. This is a total of six dives for the two certifications.
In general this would cost the shop a little more money since they would pay an instructor to be present for a total of six dives with you. However, by combining the courses they only need to pay the instructor for four dives. This is why the shops combine the classes.
My suggestion is take only the open water class. Then take an advanced class and group Nitrox with it. I agree with Steve B., get the basics down before you tackle Nitrox. You can take your advance class right after you complete your open water training.
Hope this helps,
John T.
PADI Master Scuba Diver Trainer