My understanding is that the more the latex is stretched, the more air it takes to make the sounds, and the louder the call is...
A softer stretch should also be easier to blow. I have found that a tighter stretch gives less leeway for error, but gives a bit more versatility...
I just bought some calls from Gooserbat, and he spent considerable time on the phone discussing which calls would best suit my needs... When all was said and done, I am guessing he made about $8 per hour...
Bought a box call from Paul's Calls, and he also spent some time on the phone discussing what he felt would be my best options... You might consider purchasing a couple calls from a custom call maker, if not just to talk to them and take their recommendations.