I have used Redgate SQLMonitor, Quest Spotlight and Idera Diagnostic Manager at various sites and found them all to be competent - though the user interfaces could have been more responsive for all 3.

However I haven't seen the latest versions so can't comment further.

I saw a demo of SQL Sentry Performance Advisor last year and was VERY impressed, but I have yet to see it in a production environment.

I suggest you get demos of everything, and some offer free trials. I guess that cost is probably not an issue as you have just one server?