I dont think languages differ most in prospects for success <i>after</i> you've reached functionally-complete-and-relatively-bug-free stage. The biggest difference is how do you get to this stage.<p>For a medium-to-big sized web application it would take you years to implement it in asm ;)<p>A language with a good web 'community' and great framework can be quite a big boost in getting your 1st version out to the users.<p>Also, many times, the choice is not so much between the tech but rather between the teams (i.e. when looking for outside help with development) in which case the tech matters less the people's ability to predictably hit the target on time and budget (talk to prior customers).<p>When doing internal development, if your team knows any web language well then thats what they should be using. On the other hand if the one they know is something like cold fusion I'd say - switch ;)<p>If you do switch, keep to the open source stack, linux, apache/nginx, mysql.
Again, there are quite a few people advising postgre over mysql, but I'd say it depends on how your team knows them both. if you are new to db, then you might as well start with postgre, but if you already know mysql, stay with what you know.<p>Bottom line, stay with what you know unless what you know reeeally suck for web dev, in which case switch. I'd recommend Ruby on Rails, or if you allergic to Ruby - Djungo ;)