The Boxster was the main competitor to the Z4. They are two different approaches at the same product - a rear wheel drive Roadster.
What you are saying is like trying to argue that the BMW 3 Series and the Audi A4 do not compete because one is front wheel drive.
Yes that's exactly what I am saying, the fact that the BMW 3 series and Audi A4 are powered at different ends would be a major factor in my decision making process.
As you say different approaches to making the same product.
I am not somebody who is particularly concerned with steering feel but even I noticed how poor it was. I don't feel it really communicated at all and the weight you get felt artificial. It was the first attempt BMW made at electric power steering and it showed. You could argue the steering feel in an E46 330i saloon was better - which isn't what you want from a Roadster, is it?
Strange car the Z4 I grant you that. Some say BMW can't make sports cars.
Personally I don't think the steering is anything like as bad as it was made out - JayEmm reflected these opinions in his recent review of the 3.0si Coupe. I did however link the Z4M which uses a hydraulic system.
To understand a Z4 you have to grab it by the scruff of the neck and throw it up the road, at which point it really does come alive in a manner that totally shocked me. They are quite an intimidating car to drive especially for an inexperienced driver, again not unlike riding a motorcycle.
The Porsche on the other hand has far more finesse.
The BMW is more of a challenge only because of its shortcomings, not because it is a purer drivers car. I found the suspension was too harsh at times which made the handling a bit more difficult if the road surface wasn't perfect.
I like the Z4 - it's a good car despite its shortcomings - but other than this thread I have never heard it called a better drivers car than a Boxster or a pure drivers car. I notice you ignored the part where I pointed out how full of electronics it was...
Yes but a motorcyclist will understand this, many 'sports' classic sports cars are a handful.
The Z4 suspension can get tiresome I will agree with that.
I did also recommend a Cayman - 987.2 in particular (my brother-in-law) has just bought one. My next door neighbour drives an Alpine A110GT, I like both cars.
I guess it's easier to control the ITBs with electronics than it is a complicated throttle linkage system and I totally understand that. (the N52 powered car doesn't even use a throttle, it uses variable valve lift) I really cannot fault the engine or the throttle response.
It doesn't really matter which car OP chooses, but there are some glowing reviews out there for the Z4C and Z4MC (I would certainly choose one over the overrated IMO S2000). Even the S2000 forum suggests this is the case in many places.
Sometimes it is best to ignore the crowd (especially the internet) and choose what suits you.
Anyway good luck OP, whatever car you choose.