I need low development cost because I'm a small compagny or my this year budget is small, and rapid on the market because I'm in the hurry. None of the solution is matching my needs, but Microsoft looks the best in this situation because it's the only one that will allow me to get something running soon (so something to show) with the small buget I have !
That's the right solution if your company is rich, but you cannot get more money from your boss until the system stops working.
Now, if you are in a hurry and have a small budget, but know that it will not grow massively, then what you should do is reduce the services that the custom application will bring right from the beginning. This will solve nicely over time.
In such a case, the final decision between Debian+MySQL+PHP or Pliant will come about how poor you are. If you are mostly in a hurry, and ready to spend quite some money on the support, then select Debian+MySQL+PHP, but if you are mostly poor, select Pliant because you will be abble to master it yourself much faster.