In January I sold my 07-335i 6mt sedan and bought my 02-M5.
During the decision making process I too thought about the whole warranty/service thing.
From a financial stand point, I owned my 335i out right (no loan), and was able to pay cash for the M5. Paying for repairs does not bother me. However, if you will need a loan and are not as set financially, the new car with four years of warranty and service may be a better bet.
As far as my decision, I enjoy my M5 more than I did the 335i.
M wife put the icing on the cake, so to speak, last weekend when riding with me in the M5, she said that she did not miss the 335i in the least.