Some of our friends (University students) are trying to develop a twitter type application, I want to plan for at least 1000 transactions per second (I know it's wishful thinking) for initial launch. This involves several people connecting and getting updates and posting (text + images) to site. In the back end db will server the data and also calculates rankings of what to push to user based on complex algorithm on the fly real-time.

Our group is familiar with Java and Tomcat/MySQL. We can also easily learn/code in PHP/MySQL. What is the best suited platform for our purpose ?

Though Java seem to be easy to implement for us I am afraid that hosting will be a bit difficult. I could find cloud based php hosting services (like rackspace cloudsites) at reasonable cost. Amazon EC2 is a bit over our heads to manage on day-to-day.

Also any recommendation on hosting ? (PHP or Java) We don't have millions in seed money but about $20K to start with.

Any advice on above or any thing in general approach is much appreciated.