When I built the stand for my 100g, I used 3/4" plywood over the all 2x4 frame with 1/2 foam insulation under the tank, like with a sump. Personally, I think the way the stand is constructed may be more important that whether or not you use 2x4 vs 2x6's for the uprights.... Hard to explain, but here is a picture of mine,
http://members.cox.net/allthrtl/pictures/standframe.jpg . Notice how the uprights are placed, vs. the GARF method.....Does that make sense?