Since I have an 06 M3 I was thinking of getting a Cayman S or used 997 instead of an E92 M3 when the time comes. Not because I don't like the M3's, I do, I just figured that the P-car would offer a different experience. I have 3 kids so the back seat in the E92 M3 would be useless for all 3anyway. I've read so many good & bad things about 997's so I don't know what to think about that, the back seats would come in handy in a pinch. I've read that the Cayman is great but should be better and there is no back seat at all. I like the idea of the mid-engine and personally I think the Cayman looks great, it looks like a mini-exotic to me. With the E92 M3 I'll know what I'm getting but then again it's just going to be a more powerful, heavier version of what I have now so the prospect of getting one doesn't get me as interested as one of the P-cars. With my E46 M3 I could fit my 3 kids in the back but the E92 only seats 2. I'm not that crazy about the E90 M3 for some reason. I think I'd rather just get the 335i and save a few bucks if I went for a sedan. The Sedan just isn't as exciting as the thought of owning a P-car or the E92.
So to answer your question I guess I'd keep the Cayman S over the E90 M3 and possibly over the E92 M3 as well.
07 E92 335i Saph/Black 6MT
06 E46 M3 CB/Cinn ZCP 6MT - Sold
04 E46 330i Saph/NatBrn ZSP 6MT - Sold