I've been running a mobile app development company for the last 6 years. We've found the most reliable option is AWS (Amazon). Its got lots of options for scalability, load balancing, server alerts etc.
It really all depends how quickly you scale, even when you get out of BETA, you may still be fine for a short while.
However, if you are planning to do a large advertising/marketing push, I would make sure you are on Amazon before doing so.