There are thousands of professional film editors using both the MBP's and iMacs to do professional rendering. If you do a bit of research into the professional studios and inquire about their equipment and their experience using it, you will find this is true.
BTW, whatever Mac you choose to protect your investment it is wise to buy AppleCare. This extends the warranty to 3 years and also provides 3 years of phone support. Just ask yourself, why would Apple manufacture any machine that they offer a 3 year warranty on to only last 3 years? AppleCare on an iMac can be purchased for as low as $112 if you do a bit of shopping.