Don't mind wading in here even though I work for Redgate.
You'll want to focus on two things when you're evaluating the tool (from a tech perspective). Does it give you the information you need in a timely fashion in a way that you can understand, and, does it do all that in a way that works well within your environment. The different tools largely display some of the same data collected in similar ways. The differences are in how that data gets displayed and more of the details of how the data is collected.
Of course licensing and cost play a factor, but I tend to focus on the nerdy side of things.
If you have questions about SQL Monitor, I'd be happy to get you answers.