While the S3 is still on the market, I see it as a better deal unless there is just some feature the S5 has that you must have. Comparing the S5 to a DSLR is apples to oranges. They are very different with good and bad things about each. For example, the DSLR image quality is without any doubt much better, but it would not exactly be known as compact and easily portable.
As for an upgrade from a S2 to S5, I would save the money if you are considering a DSLR in the future.
Kevin