I've used "Pre-Rinsed Play Sand" from Lowes/Home Depot for my Geo tanks before and it worked extremely well. It was under $5.00 a 50 lbs. bag so it wasn't expensive if you decide to go a different route. Lowes had the "lighter" color and the grains were finer.... and the keyword is "Pre-Rinsed"!
I would go with the 3M Color Quartz, I like the T grade, as it is not as fine as the S, The S IMHO is best for Gymno, as they are smaller, but just get the 3M, both grades are great, just depends on your filtration.
Can head into pool stores and get pool filter sand as well, it looks pretty natural - is fairly clean (needs a rinse) and the geo's will munch through it.
I agree with Darth Pike. The one I had (just gave him away a week ago) was in a 6' tank with black 3M ColorQuartz sand, he used to start at one end of the tank and shovel all the way to the opposite end making a tunnel the whole way. It's real soft sand that's uniform in size.