Writing it in PHP using CodeIgniter will give you the most hosting flexibility. It is as good as a framework as it gets and you don't have to learn any of Rails' conventions which can and do tie you into your tech.
CodeIgniter's conventions are very light and their documentation is next to none. Rails is very, very bad for people with tight budgets just like Tomcat. You need at least 512mb of ram just to get a site up for any basic Fast-CGI language.
So unless you use Google apps for your email (have full Name Server control of your server) and can get away with using SQLite or a ISAM Mysql install you will run out of memory when your site gets popular. And at that point you will have to not only increase your hosting environment but rethink your database solution that you started off with because of memory limitations in the first place.
You could go with something like:
nginx, php 5.3, google apps for mail, mongoDB for data backend and run a 64mb memcached server on a 512mb virtual private server for about 20-25 bucks (Linode.com is my recommendation). Run a codeigniter site and you are in business.
You'll even get a hardened php install at linode because you can choose Ubuntu server.
You could do the same with rails but you'll need to kick it up to do a Phusion Passenger setup to get any speed out of it. Also, ruby's garbage collecting is a joke. The best (read that as solid and scalable) rails sites run on JRuby.