I too will be in San Diego in May, since I live here! 
You are asking where the best riding in So Cal is. I'm surprised there haven't been oodles of shameless trail promotion, and trail bashing.
I recommend checking out some of our local trail websites. socalmtb comes to mind.
Go ahead and PM me, and I'll give you a long-winded answer, along with some follow-up questions.
As for a rental, I wonder if you could get one of the Competitive Cyclist demo bikes, and use it for the week, then send it back.
Regards..
You are asking where the best riding in So Cal is. I'm surprised there haven't been oodles of shameless trail promotion, and trail bashing.
I recommend checking out some of our local trail websites. socalmtb comes to mind.
Go ahead and PM me, and I'll give you a long-winded answer, along with some follow-up questions.
As for a rental, I wonder if you could get one of the Competitive Cyclist demo bikes, and use it for the week, then send it back.
Regards..