I should clarify that I'm burning their buck in a stoker with a much bigger feed tube than is on a 520. The manual advises against burning buck in a 520, but many people do with no problems. Lehigh's buck isn't real big, so I wouldn't expect it to be a problem. Before buying a TL, though, you might want to get a sample to test. Terry at Lehigh is very cooperative - you might want to talk with him about that (I think there is contact info for him earlier in this thread).
I'm not aware of Lehigh having a different price for rice vs. buck. I've taken 2 TL's and have a neighbor who has taken one, and I'm not aware of any problems with their current product.