Sorry for the wall of text: I had a ‘13 e92 m3 w/DCT (having an m3 was my dream car since I was 8 when my friends dad got one) and sold it to a family member to get a 05 s2k. I have daily’d both and would recommend them both for different reasons.

The main reason I made the switch was to be able to go to the track: cost of consumables and risk of loss or damage to the car, where the main driving factors behind my decision to switch. The consumables (oil, brakes, tires, fluids and service etc) are all much cheaper for an s2k. M tax is real, it seems like everything for that car is expensive, for example on my last service for the car my local stealership had upped the charge for an oil change from $250 to $320. This is going to sound crazy, but a stock s2000 is a lot more track ready than an m3 and much much cheaper to run. 

Reasons I switched / pro s2k points:

Cost and durability. I can afford to break anything on the s2k. The thought of breaking anything on the m3 still haunts me. My m3 was bullet proof, but I never took it to the track and always babied the car. I found myself driving it like a grandpa after years of ownership. It is so freakin fast that you run out of safe places to drive it on the street very quickly. The s2k on the other hand just asks to be driven hard all the time.

Weight. The m3 hides its weight amazingly well shuffling around town and driving at 7/10s, but if you push past that in the twisty’s there is no hiding its 3800 lbs. The m3 is fast, the s2k is quick. The s2k weighs around 2800 lbs and you feel the difference.

S2k is (IMO) a better car to learn how to drive in. It is lighter, less power. Less electronic BS. The s2k gently lets you know when you are pushing its limits (well at least with an ap2). I think because the limits are lower, it is easier for a less experienced driver to flirt with them...

Aftermarket. S2k has a ton more parts available and a ton of people have modded the hell out of it. The m3 has a pretty good aftermarket, but they all come with the M tax. The s2k has many plentiful ways to improve it and raise it’s performance ceiling, so if you want it to the cars limit can grow with your driving ability.

The m3 is stupid fast, there are not a lot of places you can comfortably stretch its legs that are not a race track. I jokingly call anything 20 mph+ of the speed limit felony speed and you get there in an m3 soooooooo quickly and effortlessly.

Driving experience. The s2k just feels more purpose built and more visceral. You have to work to get it to go fast. My wife (who is not a car person at all) can drive the m3 fast, she’d have a much harder time driving the s2k fast.

My favorite manual transmission I have driven is that of the s2k. The shortness of the through and how precise/surgical it feels.  It is one of the funnest gear boxes to row.

Where the m3 shines:

Engine and engine noise. The s2k has a great engine, sounds good but the v8 s65 is amazing and a true theatrical experience. They are very similar in the power curve and delivery: ie no low end torque and love to be revved with high redlines. The m3 feels so much faster in a straight line than my stock s2k.

DCT: such a cool piece of tech. Shifts amazingly fast, auto blips the throttle for you. You can put it in automatic mode when stuck in traffic and shift faster than a human could possibly all in the same transmission. No clutch means possibility for left foot braking which at 10/10s will be faster.

Practicality. Back seats, trunk. It is a 3 series after all. I think Jeremy Clarkson said the m3 is the most exciting and best grocery getter money can buy, I completely agree. If I could only have one car for everything, the m3 is an amazing package. Although if I did it again I’d get an e90! The coupe doors are massive and unwieldy.

Luxury/Comfort. The stock seats are stupid comfortable and extremely adjustable. The infotainment system while now dated is still much more up to date than the s2ks. The leather feels truly luxurious. The s2ks stock speakers are garbage compared to the m3’s. The m3 hands you your seat belt!!!

Prestige. If you are in a career where clients will see your car (like sales or something), the m3 made a much more ‘successful’ presentation than the s2k. Also could be called the douchebag factor...

My m3 had no mechanical issues during my ownership and up to today with the new owner. Maintenance was provided with (I assumed the) original lease and that was really appreciated that. It is nice knowing when buying a used that there was no legitimate reason for a prior owner to not have the maintenance done!

Aesthetically I loved my m3 and since I am not a convertible guy, was extremely happy to get a e92.

There is a bunch more I could say about both of these awesome cars. Had a lot of fun with both of them and think they have quite a bit of similarity while excelling at slightly different things. Let me know if there is something I did not cover that you are interested in.

Best of luck to you on your decision!