I'm writing a script to be run as a cron and I was wondering, is there any difference in speed between the Ruby MySQL or Python MySQL in terms of speed/efficiency?  Would I be better of just using PHP for this task?
The script will get data from a mysql database with 20+ fields and store them in another table every X amount of minutes.  Not much processing of the data will be necessary.
Just pick the language you feel most comfortable with. It shouldn't make a noticeable difference. 
After writing the application, you can search for bottlenecks and optimize that
Python MySQL vs Ruby MySQL