For me its actually 4 things that I look for. Internet Experience, Uptime, Service and cost.
Cost is the easiest one to put in a table and categorize. The rest are not that easy.
Budget wise, I can push for 512, but not higher. It really becomes to costly for me then.
Internet experiences
Not all service is the same. The problem is how to compare the services. I use lots of RP chat servers and sometimes online gaming. Most of these are not "priority" connection. Each ISP has their own type of prioritizing. I found that even what is use to prioritize impacts your experience.
Thus it's near impossible to see what is best for you.
Uptime
Uptime is also important. I had good uptime with MWeb, except when the under sea cable was faulty. My internet in on 24/7 and only disconnect when I reboot the systems.
Service
I had good service from MWeb, granted I have not phoned them a lot for problems.