@Amine depends on what speed and what you need the service to "do".
Gaming / Streaming or remote work (yes I know we want the Internet to just work, but some networks are better at some services - just the nature of the Internet).
I am on Metrofibre (Centurion area) on 50/50. Gaming (Fortnite) without issues, downloads in general pretty fast (55Mbps on 50/50 account) and has NEVER had an issue streaming Youtube/Netflix with Cool Ideas.
Some international streaming / speeds are not "full speed", but everything local is pretty much line speed (or higher).
But I have had some issues, which Metrofibre eventually fixed (was more a metrofibre than Cool Ideas issue, but you have to work through Cool Ideas to get things resolved).
So
1) Depends on your area (maybe do a search for "Your Area" problems
2) Unfortunately most people just complain on these forums... but you will get a general idea by browsing some of the forums
3) Phone each of the providers and get an idea of "you are number so and so in the queue) to get and idea WHEN (not if) You will have to phone them in future to solve an issue.
No ISP is perfect (also for the same reason Fibre Network will always be plain sailing)... But Knowledge is Power..
Read the Terms and Conditions on each provider.
a) Most seem to want the Router back if you cancel
b) some also only allow DEBIT orders (extra cost maybe on your bank account)
c) Some have online chat on their websites (helpful sometimes)
Good luck!