Well, I would say rigging placement (full, 7/8 etc, most widely used is 7/8) and rigging type - the "normal" type as you see in roping saddles ( and on his website) flatplate or inskirt rigging. On his economy site the top 2 saddles have an inskirt rigging - I recommend it . Much easier to use/move your leg!
Think is, you would need to talk to him what he wants you to do so he can get a tree with the right specs for your horse - gullet width for once, bar angle for another .... Maybe send him pics of your horse taken from different angles : profile, more from the front and more from the back so the shoulders of your horse can be seen and the back ,too .. or you already know what these specs are for your horse(s).
And think about what tree you like to have - I mean the tree name which really only referes to the shape/design of the swell, and what horn. If you are not familiar with terms , maybe google for "saddle tree" or "saddle horn" to get an idea - or go look at some saddles and/or or send a pic of a saddle of which you like the seat shape, and /or the swell shape and/or the horn shape. If it is a custom saddle you want he should be able to do what you want. You just need to describe it...
Also think about seat size and shape etc etc...again if need be send pics.
Either take the tree from the tree company he uses or ask him to order from another place - which will increase the costs and probably the wait time.. Learning about trees can be quite time consuming...
Or google for "custom western saddle maker" - they have very different price tags and the wait period is 6-9 months roughly speaking but will help you with what I outlined above - and use a better tree maker then his normal tree maker.
Completely confused ??
If not or to learn a little more about trees just go to the search button on this site and look for "tree" or "saddle tree" or "seat shape" or....
Enjoy the weekend
Oh, I nearly forgot: Ask him to put in the holes in the stirrup leathers at "half hole distance" and to re- enforce the "hole line" with a strap of leather. Then you can adjust the stirrup length much better.
Hope this helps a little
Tosch