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),
The company I work for (www.alertsite.com) provides basic site/service monitoring that is great for measuring basic infrastucture performance like: DNS, TCP conn, First byte, and HTML content.
We also offer measurement from the web browser that shows an incredible amount of detail about every aspect and object on the page. We call this service DejaClick and it is available for free on the desktop (www.dejaclick.com).
DejaClick is great for transactional monitoring that can visit your site, run a search, view information, login, etc.
DejaClick monitoring for a web application provides the most authentic view possible of website performance as it is measurement from the web browser.
Ken Godskind www.alertsite.com
Most frontend optimizations will help everywhere, so as ericb says, do those first. We turned on a CDN (Panther) and used this service and some others from Gomez to test the results.