I have opted for ecommercetemplates for my new web store. They have 2 option either php or asp. I have done some research and have found php is the most popular but search engines have problems with some of the generated pages. Hosting is not a problem I have either an Nt server or Unix server option available. My current store is hosted on a unix server and i find this to be a more stable platform. Doing a quick tour of the various live web sites created with ecommercetemplates the asp sites appear to have more pages cached by google?
Google has no problems indexing pages generated by PHP, unless of course you're included the session ID in the querystring I would opt for PHP