Either design will work fine.
2x's should be sighted for straight, preferably the kiln dried, and usually a good lumber yard will have better quality lumber than the box stores, but I've built many stands out of 2x from all sources over the years.
Ply ripped properly will be more accurate, IF you know what you are doing and able to make perfect rips, but really 2x is probably easier and more forgiving for the average DIY guy.
All my stands get a 3/4" ply top which helps alleviate any minor deviations, and SCA's trimless tanks will require full bottom support, so you will need a ply top, and I'd suggest a ply bottom as well.
I can post the template for you that has been the guide many have used for a basic 2x stand frame, and I have pics of my own personal design as well which is pretty similar.
http://www.reefcentral.com/forums/sh...stand+template