There is a big difference between the 300M and current 300/300C models.
The 300M was the previous 300 model from 1999-2004. It only had one engine option: 3.5L V6. It was front wheel drive.
The current 300 series is a completely different animal. It debuted in 2005 with a light refreshing in 2008. It is a rear-wheel drive sedan. It is available in numerous configurations:
The base 300 has a 2.7L V6.
300 Limited: 3.5L V6
300C: 5.7L Hemi V8
300 Heritage: 5.7L Hemi V8
300 SRT-8: 6.1L Hemi V8
Seating, materials, and luxury features also differ between each model. The base 300 is the cheapest, but many people are dissapointed with the small V6. The 300 is a big, heavy car and needs a big engine to move it down the road. I'd recommend going with the Hemi V8.
Visit www.chrysler.com for more information on the 2008 and 2009 300 models.