Back in 1991, I paid $33.5k cash for my new Q45 - a heck of a deal (MSRP was $39k). In 2003, I paid $34.5k for my new M45 - another great deal (MSRP was $43k). I am very happy, including considering inflation!
I have owned a 1991 Q45, a 1995 Q45 (bought used in 1998) and now have the 2003 M45. I definitely regretted trading in the 91 for the 95, but there is no doubt in my mind that the M45 has easily shown me why I bought the 91 Q (when I was actually shopping for a 300Z Turbo)! It is fast (faster acceleration than the first gen Q), has a butter smooth and quiet engine, and better handling.
Yeah, the M45 has a slightly lower top speed than the first gen Q45, but how often do you really get a chance to drive at those high speeds? Even in my 91 Q, I only hit 150mph on one (long) occasion - for details, see the thread on Infiniti Speeding Stories. At anything less, the M45 is an easy equal - getting to 120mph is a blast!
Every time I drive the M45, it brings back the same grin on my face that the 91 Q did for me.
So, for anybody considering getting the current gen 2003 M45, I recommend it heartily as an easy replacement for the first gen Q!
Z