In theory the maintenance on the 1 series should be free for the first four years, but I agree with larryn about sticking to BMW's maintenance schedule.
Your oil should be changed more often than 15000 miles, especially after the initial break in. And the idea of "lifetime fill" on the gearbox & diff is just plain foolishness!
My new 135i will get it's oil changed every 5000 miles & have the trans/diff changed with Redline oil every 30000 miles as well.
So to answer your question, the M3 will certainly cost a little more to maintain, but the 1 series will not be entirelly free if you choose to do things the right way.