This would probably be my preference - Some of its options include the number of simultaneous requests, delay between requests, and number of times each user should make a request. Most importantly though, it lets you specify a URL list which it can be setup to access at random - this will provide a more realistic test than simply hitting the same URL multiple times.
This is part of the httpd-tools package (i.e. you don't need to install all of Apache if it isn't your web server of choice). It lets you specify number of concurrent users and total requests - but only accesses a single URL. It provides a good breakdown of time per-request.
httperf is another choice - it tends to be better for lower concurrency tests.
If you are testing between instances you may want to have a baseline bandwidth number - iPerf should be able to provide that.
Keep in mind that only bandwidth in the same availability zone is not charged.