Im looking to buy my mom a car and Im saying there both great cars. But the Mercedes is beautiful. The prices are not the problem. My mom said she wants the M5. But which is a better car?

If you mean the 2010 F10 M5, it is not released yet.If you mean E60 M5, it is a great and very fast car.It is faster in straight line and in-gear performance than the 2010 E63 but the new E63 beats it round the track and is more comfortable to drive.It is up to your mom to decide which car she loves best.I can only tell you that M5 is nicer to drive and E63 is more luxurious.If you tell me what is your priority in choosing a car i can guide you more about these two cars.
I myself prefer M5 to any other sedan in the world because of it's performance and it's great V10 sound.
8250RPM redline!!!! fantastic to drive.