I can tell you ten different reasons why I would get the M3 over the new and improved 335is, but everyone is different, and you may be better suited for the 335.
Also, I highly doubt that the LCI version of the E92 M3 will offer any performance bump over current specs.
Here's why I'd take the M3 6 days a week and twice on Sundays:
1. V8 at 8,300 rpm. If you haven't felt this rush in the M3, you have no basis for making the comparison. It is an exhilerating feeling that the 335 will never be able to replicate, regardless of the 335is's increase in power.
2. M3 suspension. It's my understanding that the 335is is not getting a significant suspension upgrade. The M3's suspension and EDC is one of the things that helps the fat pig of an E92 defy physics and handle like it does. Without this, the 335 is just a wallowing fat pig. At the same time, the M3 also feels much more solid and much less "crashy" over bad roads.
3. M differential. Without a true LSD, traction is seriously compromised. What good is all that power if it can't be properly put down to the pavement? Not long ago, I went on a drive in the Texas hill country which a bunch of M3, 335, and 135 owners. It was great fun and I was impressed with all of the cars there. The one disadvantage of the non-M cars that stood out to me was the problems they had accelerating hard into turns. This is a disadvantage even in daily driving, when opening up the throttle in turns.
4. Exterior appearance. The 335, even with the M-sport fascia, is such a flat and generic looking car, especially from the side. Don't get me wrong, the 335 is a good looking car, it's just not special. The M3 looks infinitely better and more aggressive from every angle.
5. Exhaust note. I'm pretty sure this doesn't need any explanation. Interestingly, my E46 ZHP had a better exhaust note than the E92.
6. M-Drive, MDM, etc. The amount of adjusting and fine tuning that the M3 driver can do to adjust throttle response, suspension settings, steering effort, and traction control to fit his personality and the current driving conditions is amazing. The fact that one can transform this car from a docile and comfortable daily driver to an absolute track beast instantly with the touch of a button is incredible.
7. M reputation/cache. No, women don't immediately recognize the M3 for what it is and fall over themselves trying to talk to you, but the M3 (no matter what generation) is respected in the enthusiast community as a driver's car and gets special attention. The 335 does not and never will have the same pull.
8. Much more track ready. This factor is sort of captured in the above reasons, but worth a separate post. If you want to track your BMW, the M3 (aside from a recommended, but not necessary, brake fluid and pad change) can be driven straight onto the track and driven hard. Obviously, the 335 can also be tracked, but its weaknesses are quickly exposed and its not nearly as rewarding of an experience.
9. Only V8 3 series ever. The E9X M3 almost certainly will be the only factory V8 3 series ever produced. Will that make this car a collectible? Not any time soon, but it does make it a very unique and desirable model.
10. Leather. The M3's leather is vastly superior to the leather in the 335 and much softer and more comfortable to the touch.
11. No HPFP. 'Nuff said.
Really, there are only two downsides to the M3 compared to the 335is: initial cost (which I suspect will be less than $10k more when similarly optioned) and fuel efficiency, which will likely only be a few mpg different (I get almost 17 mpg in the city and 21 on the highway), and only a few hundred extra dollars per year.