woh ok lets start with the results... neither asp.net nor php will blow anything out of the water with execution speed, you should be more worried about coding speed.
the server and setup will be the deciding faction in which executes faster.. a winXP IIS server and asp will execute code faster than a winXP apache using cgi-mode non-mod_php
if your developers are better at coding with asp then go asp, if your developers are better at coding in php then stick with php... if you know php now its probably not worth the time to learn asp
if you aren't really programmers at all and just kinda know enough to get by, and would'nt notice the difference between them, then you should choose which ever one makes the higher ups the most happy and stick with it
i think php's syntax and function calls are more better like than asp anyday, asp is just more confusing in my opinion, but then again i know more php than asp so im biased to begin with