You should check out the Air software's Strike 2. It was on sale for only $43 a while back, not sure of the price right now. AD2 is OK but you have to purchase all the sounds. It comes with a skimpy library. Strike2 comes with more samples that anyone could possibly use.
It works differently than Session drummer so you have to read the manual.
But it took me no time at all to start using it and now it's my main drummer. I will probably not use AD again.
I still use Session Drummer however because I sampled my own drums and SD makes it easy to make a kit out of my own samples. I think Strike will do the same but haven't had time to dig that deep.
I have never had an issue with SD loading. I've had it on more than 2 dozen installs of Sonar.