I would go with PHP. At least if you do it that way, you can run it on multiple platforms where ASP.NET is limited to Windows .NET.
PHP can connect to a multitude of databases as well.
PHP 4 | PHP 5 | ASP.NET
Software price: free free free
Platform price: free free $$
Speed: strong strong weak
Efficiency: strong strong weak
Security: strong strong strong
Platform: strong strong weak (IIS only)
Platform: any any win32 (IIS only)
Source available:yes yes no
Exceptions: no yes yes
OOP: weak strong strong
Price: Here, we must consider not simply the price tag of the initial investment, which, in the case of PHP, is obviously free, but also the implementation, maintenance, and debugging costs. In the case of PHP, you may invest in the Zend optimization engine. With ASP, however, you're investing from the very beginning, and you're spending for add-on technologies—libraries for doing graphics manipulations, for instance. But, in the long term, PHP isn't going to press you to upgrade and collect more licensing fees. Everyone who has dealt with complex licensing also knows that companies spend time and money just ensuring they are compliant. Furthermore, you have a difference in response when getting bugs fixed. This, of course, translates to time, which translates to cost for overall development.
Speed and efficiency: ASP.NET is a framework allowing you to use various programming languages. In addition, it is touted as having a great object-oriented model. All this is true, but it becomes a detriment as far as speed is concerned. For all that advantage, there is a lot more code to run through to execute the same ASP page than you have to execute in the PHP engine for an equivalent PHP page. PHP is the quick-and-dirty type of solution, the one to get the job done. And though a lot of robustness has been added to it since its 2.0 and 3.0 days, it still retains that core optimized high-speed approach.
Speed is not the only consideration. Memory usage is also important.
Security: ASP.NET runs on IIS, which has been compromised innumerable times, as evidenced by IT news reports every other week. It has become such a liability, in fact, that in spite of all the marketing dollars spent on it, many IT professionals refuse to have their networks exposed with an IIS Web server. PHP, however, works with Apache, which has a proven track record of speed, reliability, and hardened security. Check
www.securityfocus.com for more information.
Cross-platform applicability: ASP.NET runs on IIS and is starting to run on Apache, which can run on a whole host of platforms. PHP has been designed to work with Apache from the beginning, so you have many proven and reliable server platforms to choose from.
Open source opportunity: Open source is not just some philosophical torch idealistic programmers, or companies wanting to save a few bucks on licensing costs, are carrying. When you're dealing with bugs in the software itself, open source can be a serious godsend.
In either case, with PHP or ASP.NET, you have a large user base using the software and possibly encountering bugs. With ASP.NET, those bugs have to go through a bureaucratic process to get acknowledged, fixed, tested, and rolled out in a new patch or release. PHP fixes, however, can get fixed quickly and rereleased. Anyone who has watched open-source development knows new releases and patches often come out in days rather than in weeks or months, as with commercial software. If that's not fast enough, you can always fix a problem yourself if you have to.