No, no, no.... that isn't what Rick meant. 60 fps is a non-standard frame rate. Almost always -- think about 99% of the time -- the frame rate you really want to use is 59.94 fps. No kidding.
Now, if you're just farting around with AE, use any frame rate you want. If you actually want to use your AE work for something in a different application, you need to keep such things in mind. Frame rate is one item on a list of items you must technically meet known as delivery specifications.
And speaking of delivery specifications, 640x250 is a very weird resolution. Perhaps you can elaborate on what will happen with your AE work when you're done with it.