All Mumble hosts run the same free software. What differs is the service around it. Check these before you buy.
Slots, not channels
You pay for how many people can be connected at once, not for how many channels you create. Count your peak, not your member list: How many slots does a voice server need.
Location and delay
Voice is sensitive to delay. Pick a server near most of your group. A server in the same region as most players keeps delay low enough that people do not talk over each other.
Control of your own server
Look for a join password, the SuperUser password and a way to edit settings, without having to ask support: How to set a join password.
Voice servers are targets. Look for network protection against floods, not only a promise of uptime.
Clear pricing
Check what a slot increase costs, whether there is a setup fee, and how refunds work. PlayNHost voice servers start at a few dollars a month, with a 72-hour money-back guarantee and no licence fees, because Mumble is free software.
Support
Ask who answers when something breaks, and how fast. You want a person, not only a knowledge base.
What you do not need
Special hardware, a dedicated IP address or a domain name. The address from your Overview page is enough, and you can add a dedicated address later if you want a nicer one.