Tips for Picking a show lamb?

Personally, I would probably go with the younger lamb, especially since you don't show until August. I have found it to be a lot easier to just "pour" the feed to a smaller lamb and just let him eat however much he wants versus having to constantly measure out the feed for a larger lamb. It's also a lot easier to put the weight on, than to take it off because sometimes even rigorous exercise doesn't do much of a difference.