There are several useful QoS parameters, although not all are as easily measured as e.g. 99.98% uptime, which is easily derived - 99.98% of the time, your server is available. Other metrics:
- What sort of bandwidth do they support? How about if it's a shared pipe and they've multiple high-volume clients? You're unlikely to get an honest answer to this from the vendor.
- If you're on a shared host, what sort of competition for resources will you have on the box? Again, this is competitive information, so you're unlikely to get an honest response.
- How independent is each application? i.e. if another application on a co-hosted box loops MySQL, will your app suffer?
- How often do they upgrade dependencies, and does that affect their 99.98% uptime?
Then there are the fluffy not-entirely-QoS related hosting problems:
- Will they do your backups etc.?
- How responsive are they to queries? Do they have a guaranteed response time e.g. 3 hours from first email to first response, 24/7?
- Are they efficient? Have they resolved issues within a useful timeframe? A quick response-time is nice and feelgood, but if they don't fix the problem, that's not quite so good!
- Are they patient with you when you suggest issues, but due to your own technical background are slightly off-base? :)