Just finished a 5/4 cedar floor deck with redwood 4x4's and 2x4's,2x6's for the rails with Fortress iron balusters. The project came out very nice it was all stained with superdeck "cedar" and the cedar came out a nice pale yellow and the redwood... well it looked like redwood I like the 5/4 better than the 2x6 as it just looks nicer.
I live in a dry country so can not tell you how the 2 hold up in wet conditions. The biggest things that come to my mind is cedar tends to be straighter with less knots and slivers (in my area). But ceder only comes in 5/4 so if you are thinking of putting your joist on 24 centers the only way to go is redwood.