Get a 118d and get it remapped, costs around £300 and you will have more power than a standard 120d
Only trouble the same applies to a 120d - remap it and you will have the performance of a standard 123d and so on....
How do you rate your Spider by the way? Is it the current model? If it is, I had my eyes on one of those before deciding on my 120d M-Sport. I think practicality was the main reason for me not getting one, they are gorgeous cars.
I faced the same dilemma as you but my choice was between the 120d and 123d. I think this was an easier choice though as I wanted as much performance/£ for my buck as possible and the 120d is hard to beat. The 123d is a very fast car but you do pay a premium for it and the difference in mpg between the 120d vs. 123d is greater than the 118d vs. 120d.
Compromising is one of the disappointing features of life and I guess it all comes down to whether you value performance higher than comfort. The 118d is a very capable car and no slug, and needless to say the cost of motoring is ever increasing. That said, if it were me I'd want the extra punch over a couple of gadgets.
You didn't say whether you are buying a new or used? The current range is about to be replaced, possibly at the end of this year so might be worth holding out for. Alternatively there should be some bargains to be had if you are happy with going for the current model over the upcoming one.