There is a 'Few Variations' that will make the different between those 2 classes for casting speed.
Equiments and Hardware.
Equiments. Wand, Rod or TechMag.
Hardware. Controller. (Turbo HORI or normal PS2 and XBOX 360) PC's you use K-Board.
Has been tested in game.
AT-same level, same tech's, same equipment.
1 is using Turbo controller, other use a normal 360.
The 1st 2 cast is same speed.
By the time it hit the 3rd cast, It's about 1/3 slower on the 360 controller.
Then slower, and slower as it goes on.
Reason, Your Turbo will not miss.
So, If you question is asking for "If I cast faster, I can level my Tech's faster"
Yes, get yourself a Turbo controller.
However! If your question "Which class will out perfrom at casting speed"
I belive Syl and Magician had provided you with the the adequate advice.
I have both AT and MF at Level 20 all Tech's 50. With a 360 controller. AT is the faster.
ps: If you are worry about the low HP on your MF, and thinking of cast and run. Use a TechMag