With your testing, is it the same server?
Concatenating raw html in the code behind can big a big maintenance nightmare. Also, depending on your site architecture you may to re-compile to deploy a simple change when you can just push the aspx.
Concatenating raw html in my option would be how a class asp developer new to .net would do.
If you are using webforms, you would typically create re-usable server controls or use the ones that are provided.