I did move to XyCraft tanks (just to test them out) and they're incredibly cheap compared to RC tanks. I really don't like the way he basically one-upped another mod author by copying something and then making it loads cheaper.
I like both. I prefer smaller footprints for my tanks and railcraft tanks offer more storage in a smaller space because they include the walls in the calculation. 1.5 version of railcraft has steel railcraft tanks that double the volume to 32 buckets per block so its going to be 3x3x? steel tanks for me. If i wanted really big tanks though I'd use the xycraft ones, plus I like the liquid detector block.