All plugs should perform the same when they are newly installed. Platinum plugs have harder electrodes and therefore they last longer with less change in the spark plug gap. Iridium plugs are even longer lasting than platinum.
There will be no difference in your mpg unless the old plugs are very old and the electrodes are worn down. There is a debate about the long lasting plugs. Yes, they last but is it a good thing to leave them in the car for 100K miles and 5 or 10 years? Plugs that are in a motor a long time can be difficult to remove and may damage the threads in the cylinder heads. Some users of the long lasting plugs take them out and reinstall them every couple of years. Some just leave them in place and hope.
Best advice is to use the exact same plug that came from the factory when replacing spark plugs. Then follow the interval recommended in the owner's manual for replacing them.