TxLobo.......I stay at either the Munich Marriott or the Renaissance frequently. As the posts above say, they are both within a block of each other. Take The Lufthansa Shuttle from the airport to downtown but get off at the first stop....München Nord - Schwabing....and you only have a short 2 block walk to the Renaissance.
I have never been upgraded at either hotel and I am a Marriott Platinum. Sounds, however, like others have had better luck. I do prefer the Marriott to the Renaissance but the two are both nice hotels.
Have fun...