Hi taipan, I have also been comparing these two fantastic motor cars, not an easy decision as you will know! You will get bias towards the E46 M3 on this forum and vice versa on 1Addicts about the 135i.
As one forum member said “Life is too short to not have an M3
If you can afford it, get it.”
I think this sums it up, if the M3 is your dream car and you are prepared for the costs of ownership, i.e. service II inspection A$2500.00 and new clutch about the same, go for it. I must admit that the 135i looks like a girls car compared to the M3.
This is the video that helped me to make a decision: