As per the comments above I don't think there is a right or wrong answer and as such ideally you would run both - unfortunately reality and ideals very rarely go hand in hand so let me give you my thoughts.
Similar to you OP I was recently tempted with a good deal on an E89 sDrive 23i and now find myself with a Z3 and the E89. Probably the big difference is that unlike you the E89 is the 6th Zed I've owned since March 2020 when I sold my Z4MC - along the way I've had a 2004 E85 3.0i, a 2006 E85 2.5i a 1999 Z3 M44 and a 1999 Z3 2.8.
Now the Z4MC was everything everyone said it would be and to sum it up every trip out was an event - by that I mean there was at least one point in every journey that I thought the car would be written off. Fun yes but what I was looking for - initially probably yes but as the months progressed no. I still look back with fondness and sometimes still wish I had that car but in reality for me it was a square peg round hole scenario.
When I moved to the 3.0i the drives weren't as eventful but it did sound good and with the automatic box was a lot more sedate. However I could never resolve myself to having a weekend car with an automatic box so I bough the 2.5i manual. Never really took to the 2.5i and as a result ended up buying the M44 just for something totally different.
The M44 was great for what I wanted and the high revving engine coupled with the light body was great fun on the Peak District Country roads. But this time I got fed up having to work it so hard and missed the sound of the straight six.
Enter the pre-facelift Z3 2.8 - the chap I bought it off was in his 80's and admitted to me he had virtually stop driving it because he felt it was to much car for him. He had done very few miles in the 7 years he had owned it so it came to me in lovely original condition. It doesn't feel anything like the E85 and you sit more on top of it than in it but it sounds great and I think the looks are surpassed only by the Z4MC. So much so that I am really struggling to bring myself to part with it and if I had any history with the car I don't think I would.
However, having spent a couple of weeks with the E89 it has grown on me and the 2.5 straight six sounds great and has to be worked just enough to make it fun. The driving position is good and somewhere between sitting on top of the Z3 and down inside the E85. Equally the handling is perfect for me and much more sure footed the the Z3 and less flighty than the E85. Finally I love the versatility the hardtop affords and although not as immediately striking as a coupe it does come a close 2nd with the roof up.
So for me I guess the E89 really is the goldilocks Zed and as much I'd love to keep the Z3 to experience it's occasional rawness with the high revving 2.5 on the 19 inch rims I can still get some of that from the comfort of the E89.
At the end of the day it's all about experiences rather than things and I guess from my perspective it just feels like I can have more of those for less effort in the E89 - with the Z3 or indeed the E85 there would always be more to do to keep the good times rolling but I guess that's the joy of classic motoring.
Anyway whatever you do I'm sure you'll have fun just as soon as they let us get out on the road again.
