Check out the answer to a similar question which should help:
To answer your question more specifically however, every major monitoring service will provide performance metrics in addition to basic uptime/availability notifications. For example, the Top Tier (Webmetrics, Keynote, Gomez) will give you waterfall graphs that show you a timeline showing how every item on your page loads from all around the world.
To determine which is the best, I would recommend you get a trial with each one and see for yourself.
Note that this kind of information, especially if you are looking for performance from locations around the world, is what separates the top tier from the bottom.