Frankly this type of monitoring (geographically dispersed) is most often requested by pointy haired bosses. Measuring is great if what you're measuring is realistically under your control. If your app is slow in Outer Mongolia, are you going to upgrade the internet backbone there? Or go upgrade the users dialup modem?
Only pay the extra to measure response time from say, china, if you have a target response time in mind, money, and a plausible plan to improve it if it does not meet the target. Otherwise, spend the money on making your app faster by load testing and scaling it locally and paying for a reliable, professional host. This is not to say don't monitor--you need to watch relative response times and downtime, but the services you're describing cost a lot more, and buy you very little.
Edit: Also, I strongly suggest transactional monitoring. If your monitor plays a transactional script (add to cart, checkout, etc) as opposed to a simple page request and you monitor the returned content, you will be able to catch all sorts of errors that are missed by simple page checking. (someone kicked the database ethernet cord out, db lost connection, etc),
