You can have a paid account with PBXes.org without using voip.ms at all. But I never checked what their prices were since I already was with voip.ms and I am happy with the functionalities.
In the last 6 months, PBXes.org had 3 outages. They lasted less than an hour. They send you an email saying you are routed to another server automatically with a paid account.
When you install SipDroid, an ad banner appears: if you have already configured your phone for voip.ms, SipDroid will do all of this automatically for you. But you will have to review all parameters because it will also setup voicemail and other stuff you don't want if you want to stick with voip.ms.
I use the GSM CODEC in my phone. It is compatible with voip.ms, the sound is clear enough and takes 1/2MB per minute.
I may (or may not) have forgotten something because I setup everything about 6 months ago. I hope I didn't.