what are the advantages of stainless risers over aluminum ?

Stainless will last longer. Also most stainless risers are longer and dump the water into the exhaust farther back in the pipe. This gives a little more Hp, better sound and less chance of reversion.
On a stock engine, the only real difference is longevity.

I have stock 502's, fresh water use only. I had water in one cylinder at the of season last year. My main concern is reversion. I'm thinking of aftermarket exhaust. maybe dana.

I doubt it was reversion with a stock 502. More than likely you had a bad riser gasket.