This is more of a comparison between used and brand new. Smartly options you can push the M235i well into $50K territory. A quick perusal of your local BMW inventory should show you that used or abused E92 M3 Coupes can be found for anywhere between $30 and 70K of course depending on use or abuse. Because its MUCH better to buy a CPO car than a privately sold one warranty talk and other German car maintenance nonsense need not apply.
Passenger wise the larger E92 certainly has an advantage. You'll be able to fit 4 in the M235i as well, just not if they have legs, or a neck. However the rest of the interior has to be given to the M235i, design, materials and technology have all advanceed rapidly since the E92 M3 first showed its bonnet. While not outdated by any means for those technology anoraks the M235i is the choice.
As for driving, well lets say both are QUICK and leave it at that. The smaller M235i may be short on cylinders but it packs punchy turbos and the ability to tune, ever seen a tuned 335i walk an M3? Yea that, just in an even lighter car. Downside to the 2er is Electric Steering vs the old school hydraulics of the E92.
The REAL fault with the M235i is no DCT option, it had been avaialble with the 135i and was WICKED fun in the diminutive package (I should know, used to 'rent' one) also a tragedy is the omission of LSD as a factory option. Now it is only available as a BMW Performance Accessory and must be installed post delivery.
Personally the M235i is the one for me, the 135i was such a fantastic car and I've missed it dearly since 2010. The smaller size lines it up much closer with the E36 M3's of yore, not to mention the downright silky operation of the I6 which I'm sorry just cannot be replicated...