Anyone here used either before, or found one to be better than the other for particular use cases?

I've used both of them for VMs before (and the Amazon equivalents) and I've tended to think of them a bit like hotel room rates. The fully flex rate (or Pay as you go) is the most expensive, but you can change things round as much as you like or cancel and pay no more. The Savings Rate is the 'semi flex' rate where you can cancel and change some things (ie you can still cancel machines, but you have to move the committed spend elsewhere). And the reservations are the advance rate - cheapest, but most restricted in that you basically have to keep the same type of machine in the same region. If you're reasonably confident that the load or requirement of something isn't going to change then the reservation works well. Normally with clients we would run a new VM for 3 months or so after launching a new site on the full PAYG price to decide how it performs both day to day and for any peaks during then. And only after that consider using reserved if it is stable and predictable. Though rarely for more than 1 year as 3 years is a very long commitment to assume minimal change. Reserved also seemed easier to allocate to a particular client billing wise. Savings plans are more flexible, but, especially in an agency, that flex can also be a bit more work to allocate when charging back out depending on how good you are with the billing setup.

Thank you for the comprehensive info @CodeBunTes! This is pretty much how I understood it to be, and makes sense that if you know what you're going to use then go reservations, otherwise if you're trying a bit of everything go for savings

Do you have a rough idea of the cost saving of moving from a PAYGO to savings plan?

Azure Advisor will tell you 🙂 (found in your portal)

OK that screen had some zeros... will advise the client thanks!