What do Calcutipr and Gratuitous have in common? They are both tip calculators. Yet what seems like a simple thing shows amazing diversity on the App Store, with well over 3 dozen tip calculators available. What makes these two different from the rest? I have no idea, not having tested the dozens of other options available.
But I can tell you choosing a tip calculator has now become largely one of personal preference. Calcutipr (iTunes link), for example, has a sleek and functional interface with lots of options. You can hit a button to round up, for example. Gratuitous (iTunes link), on the other hand, is more "designed" and streamlines functions like splitting a bill. Both do the same thing, but in different ways and with different options. Tip calcs are like shoes: they all do essentially the same thing, so your decision boils down to what you like and what you'll pay. Calcutipr is free and Gratuitous is $1.99, which may make the decision a simple one for some. Both work on iPhone and iPod touch.
I don't know if you'll like either one of these, but they do the job. If you don't like them, you have plenty of other options (iTunes link).