What's faster, a sportscar or a truck ? It depends on what you are trying to move.

A load of bricks will be moved faster by truck, even though in an absolute sense the sportscar is faster...

If you are doing vector processing and have a hard-to-parallelize problem then a supercomputer is probably the only way to go.

Otherwise EC2 will possibly be faster (it still depends on lots of subtle factors, such as how compute intensive vs communications intensive your application is).

The only way to know for sure is to do a limited benchmark on the core of your problem in order to figure out which architecture works best.