The Freightliners we have owned in the past were nothing to get excited about. I only have experience with the FL70 & 80's. You will have to read some of the posts on here relating to the M2's. As you stated, we had many part availibility problems. While I am sure there are some good dealers, our experience with them was poor. Either work was done incorrectly, used wrong parts, or left our truck sitting for weeks as they were too busy, or were waiting on parts. Keep in mind this only relates to my experience and the FL70 and 80. I see many Freightliner Century trucks but can't comment on those.
As far as resale, best service,locations, ect, I would have to go with Kenworth. We have a T300 with 300,000 on it, and our cost to date on repairs is half what the Frieghtliner was. Never had to wait or go through a bunch of game playing on repairs ect.
The downside to KW and Pete is they have smaller cabs. Alot of folks run a class 8 and that might be worth looking at, but will cost you more.