The Nashbars are wider, extend further forward and have a longer straight section of bar in the "drop" position. The Nittos have nicer curve and a little wider flat section near the stem.
The main reason I think I pefer the Nashbars is the longer drop section. I have large hands and I didn't have much room on the Nittos. However I did not have both handlebars on the same bike or even at the same time. The bike with the Nittos was stolen from my garage and I bought the Nashbars as replacement on a budget.
I spend most of my time on the flat sections at the ends of the bar or on the curve just behind the brake levers. I find it quick and easy to get the the brakes from either position with the curve being the best position to use the brakes. The curve on the moustache bars is a a similar reach but a little wider than the hoods on a drop bar but it is much easier to use the brakes from that position. In addition it is much easier to get from the "drops" to the brakes on a moustache than it is to get from the top to the hoods on a drop bar. Also the hand positions on the moustache are all wider than the tops of a drop so it gives you better control.
If you are spending all of your time on the tops then you probably need to raise your handlebars. Your current stem probably doesn't have much adjustment so you might want to look at a new stem. I found some Kalloy in the QBP catalog (you can order them from you favorite LBS) that are inexpensive and have a position angle to allow you to raise your bars up. You may want to do this no matter which bar you go with.
I just know I am never going back to a flat/riser bar. Even my MTB is getting an H-bar or a Mary.
Craig