No comparison between the two the only thing I am going to miss going from an STI to an M3 is the AWD. Well and the money in my pocket
If you want a second car that is fun to drive, great functionality and good in any weather condition then the STI is a really good option for the money.
As to the engine there have been some people with blown ringlands due to detonation. The stock tune is not very good (I tuned mine to stage 1 after 500 miles). I'm coming up on 60k miles with no issues, it's been a very reliable DD for me.
The car does need a few upgrades to make it feel better (stage 1 or 2) really ups the power/tq to where it should be. Some sway bars, steering rack bushings, alignment and SS brake lines all fix some of the stock issues Ive found.
If it's going to be your main car you are really going to miss what the M3 brings to your drive.