使用.asmx文件或.aspx文件接收请求？

I'm setting up my site to receive info from people via text message. The way it works is they text a number, that service then sends an HTTP POST to a url I specify. I've heard that .asmx files are better than .aspx files because they don't go through the whole page lifecycle. However, I don't really understand how to get a .asmx file running, and can you even call it with a POST, ie, www.mysite.com/webservice.asmx? I know I can make it work with a .aspx file, but I wanted to check to see if there was a better way before I undertake this endeavor.I'm setting up my site to receive info from peo