I honestly thought I was the only E93 owner in this UK bit, I've missed you all it seems
I had the choice of E93 or E92 and chose the E93 based on its looks, put them side by side and the E93 has a lower roof line than the E92 giving a slightly sportier look which did it for me. My head said "get the lighter one" but in truth that was just the racer in me thinking of potential track time, my heart wanted the E93. I did have some criteria though, it had to be black or grey as the black rubber roof seals going horizontally across stand out a little too much on white and well polished silver cars. It also had to be an M-sport model as the SE E93 looked a little off without the bulkier body kit.
In terms of performance in a straight line, driving both in standard trims I really couldn't tell much of a difference if any (335i coupe vs 335i 'vert), it only really became evident around tight bends carrying a little speed where the E92 felt a little nimbler to throw in and drive out, the E93 felt like the rear was happier to step out of line when you threw the extra weight in the back around the corner. That said, it really is a superb handling car out of the box, it sticks so well to the tarmac and I'd imagine only around a race track would you see a difference between the two. I still love the look of the E93 M3, for me that's the ultimate (in looks) 3-Series.
I forgot about the weight as soon as I drove it away from the showroom on a blazing hot Summer's day, the experience was fantastic. Sun shining down on you, cool breeze and a beautiful engine note (for the 6-cyl anyway), she gets A LOT of attention in the Summer but that being said, those not in the know actually think it's a coupe when the roof is up so compliments still get passed in the dull Winter months. I love the feature that allows you to drop the roof remotely on the keyfob, you often see people watch in amazement in the pub garden when it transforms.
I'm now at COBB Stage 1 tune and the thing is quite honestly dangerous at times, it has so much torque that any disadvantages the weight may have given going fast and staright are probably non-existent now, I just look forward to the Spring/Summer when the roof comes off again and I get to appreciate its new performance properly.
My advice is to drive both and make your mind up. If it's a fast car that you want then you need a 335i/335d at least in either trim but that's not what the E93 is about, it's about all of the benefits that the folding top gives you. Personally if I was considering a 320d, I would have it remapped as you'd really enjoy the extra torque