Well the price difference isn't that big, although I paid 19 200 USD for my 3.0si Coupe with 17400 miles on it. If you have the money to buy the M, go for it. I chose the 3.0si because it was easily available and such cars are not very popular here and also because I wanted an automatic and the Ms do not have it. At the end of the day the M will the most thirsty one.
P.S.: Also, you will be able to FI the 3.0i and the M, but no the 3.0si.