Both cars can be made to work very well. Some will say the MO3 because they have a good setup, others will say the MO5 because it is more forgiving for new drivers.
I would go with the MO5 for the following reasons:
less prone to traction rolling, more stable
parts are easy to get and cheap
easy to work on
fits more new styled mini bodies
uses the 1150 bearings in the wheel hubs that can take a knock or two
more selection in after market parts
The MO5 will become the norm in mini and the MO3 kits will be removed from the market. The biggest decision is do you want a current product and how much you want to spend. Is the car to race or for just running around the carpark. For bashing I would get the MO3.
I still have both chassis's but I only race my MO5.
Have a read of the Mini thread in this forum and the mini thread in the aussie forum and see what others have written, then make your decision. Another place to visit and read is attached:
Good luck and happy racing,