Now then Captain. . .
Quite a difficult question. I came from a mark 2 MX5 and it was a brilliant car and much easier to drive and run than the Zed 3.0si I presently have - far more relaxing and convenient to jump in and 'nip to the shop' - something I still do in the Zed - but every journey is an event in the Zed. The 2.0 litre I had before this was an 'easier' drive but still was a 'step up' from the MX and felt very special.
In terms of costs the Zed isn't a great deal more expensive to insure though petrol costs are higher in the 3.0 litre. Tyres are more expensive in the sports 18 inch staggered option - have to spend over £100 to get adequate rubber. At the price level you're talking about, an independent specialist is a must for servicing - costs a fortune to have servicing and repairs done at the dealerships.
There are some weaknesses - roof motors can stop operating due to water and rusting of the motor (drain points need to be kept clear) - replacing this is expensive £6-700+ so finding a Zed where this has been replaced already can be an advantage. Regular maintenance (surprisingly long intervals) means that many Forum members feel that 100,000 miles is not a problem on a Zed.
There are some posts about checking a Zed before purchase which give lots of information about the Zeds and what to look out for at the beginning of the Forum.
I felt it was a big step up in moving to a Zed from an MX - I bought my first Zed because of it's looks and BMW quality and 'driving' image. I took two test drives and didn't like the way the Zed followed cambers and contours in the road - I also felt the ride was too hard and walked away. Then after another test drive I was concerned about the 2nd and 3rd gear differential (big drop in revs) so decided against it again. But - the looks still kept me interested and I could feel the potential in handling as against the MX so was still tempted - after making what I felt was a great p/x against a sports bike a went ahead and made the deal.
So you can see I had my reservations - the MX had been a purchase that was easy - a no brainer - I bought one because they are reliable and affordable and fun and I never thought much about the car except that I got in it every day and loved driving it - a wonderful fun car which I could rely upon and never gave me any problems in 6 years.
Owning a Zed is different - it becomes more demanding. It took me some time to get to know the Zed and exploit it's capabilities. It took time to get used to the suspension and ride - the steering and handling (I changed the tyres to non runflats - much better ride). The gear change is not as slick as the MX. It's a more 'serious' purchase. You have to start being concerned about tyres, brakes, brake fluid, steering geometry and tracking, as they are such a capable machine - that's why regular maintenance and history is important. A well cared for Zed is a delight, a serious purchase and very capable. It will give you a more intensive and purposeful experience - it feels a more 'heavy weight sports car' not in weight but in demands. It is a challenge to drive well and at speed but a wonderful experience. It can also be driven sedately and in a relaxed manner - but it's hard to refrain from dipping into it's true potential.
I could go on - but these are my views and experience coming from a similar starting point as yourself. There are many Forumites with far more experience and ownership knowledge than myself and I'm sure they'll have their say.
I'm loving my Zed ownership and I may return to a 2.0 litre one day and would have no hesitation in returning to another MX5 in the years ahead but for now my Zed ownership is very special and the pleasure of seeing that sportscar on the drive and the experiences it gives me when out on the road is worth the investment in commitment and cost. I think that's why this Forum is so active - that's another aspect of Zed ownership I particularly enjoy - getting out on a safe yet enjoyable run with like minded owners - come join us - you'll love it.
Take time deciding what Zed you want and be patient - there are lots for sale and many options and modifications which can be applied - make sure you invest in what you want and need. Keep asking questions on this Forum and go for it - with care you wont regret it - it is a special experience.
Hope this helps