If I were starting from scratch, I'd still look around at the team I was working with and see what they know. Platform that no one on your team knows is the wrong platform. For production work, stick with what you know, unless there is a pressing reason that you have to change.
If you are determined to learn something new, I'd suggest Ruby on Rails. In just a couple of years it has become one of the major milestones to compare other systems to: "I use xyz, it's like rails except..." That alone speaks to the kind of impact it has had on the web development community. Ruby gives you a hugely powerful server-side scripting language, and Rail allows for rapid development of high quality applications.
