I have 3 x Conway Stewart 85L vintage pens and also various vintage Waterman #2 nibs on 52's and 12's. My eyedropper Waterman has a screw cap rather than slip on cap, but still I wouldn't be carrying a vintage eyedropper pen out and about, as the simple feed tends to easily leak when jostled. But eyedropper pens, because of their simple design, have such good and wet flow of ink that I really enjoy writing with them.
If you are after flexy and expressive nib, I would definitely go for the Waterman 12. All of my CS85L pens have nibs that are made of 14K gold that is not as springy as Waterman #2 nib's 14K gold. CS85L's nibs tend to be soft, as in bendable, but they don't always bend back i.e. lacks springiness. Whereas when Waterman's #2 is flexy, it is really bouncy flex and a lot more suitable if you are after flex writing with responsiveness.
I love my CS85L's due to their form factor (size, length, barrel diameter, and weight; they just fit my hand perfectly), but I don't collect them for their flex nibs. Waterman's #2 nibs are well known for their flexibility (if you find a good one) so I'd say go for the Watermans. Just my two cents from someone who owns and enjoys both. I hope it helps.