I did not own a 335, but I did own a 135 for 3 years. I had driven a number of M3s, from stock to supercharged. The difference in the quality of the chassis and feedback to the driver is far superior in the M3. I spent several years and 12-15k easy (parts costs alone) trying to make a non M into one. Just when I thought I had it done, I had the opportunity to yet again track an M3. My car had everything from an LSD and all of the M3 suspension bits, to more power than the M3. The difference was still quite clear, a non M will never be one.
The difference starts with the basics. Change tires on the 135/335, then change the suspension to try to fix the next weak link. Then eventually you get an LSD, and then all of the bushings, ball joints, brakes etc aren't up to par. So then you get into that. In the end, you'll still fall short.
You put upgraded wheels and tires on an M3 and it takes it in stride, no problem.
The differences go far beyond V8 and LSD. Once you spend a short amount of time in an M3 you will be sold. I've been driving other friend's cars for years in all sorts of conditions. Take one for a drive and you'll get rid of the 335 in no time. The mods on the M3 are far more expensive, but well worth it in the end.
I picked up an M3 and I haven't missed a single thing about my 135 since.
Do it, you won't be sorry that you did. You'll only be sorry you didn't go with your gut in the beginning, I know this first hand!