I'm generally very methodical and deliberative when it comes to selecting a watch myself, and without a doubt, how much I want to wear it is the absolute prime criterion. It so happens I'd decided between these two on that very basis before anyone responded to the initial post. I didn't intend to "crowdsource" a decision, so much as to initiate a discussion around choosing between watches with very different sources of appeal.
I definitely get the association of Cartier with shaped cases, though some of their most interesting and characteristic features (mystery dials, jumping hours, retrograde indications, central chronograph, etc.) have been housed in the more conventional Rotonde case. I happen to be a sucker for shaped watches, which are all I've ever owned from Cartier to date, but I've also been eyeing the engraved disk on the manual wind Jour et Nuit since it first appeared in the CPCP line a couple of decades ago. As for the UN, it's actually the design, as opposed to the fascinating complications, that's kept me from acquiring one sooner.
Thanks for chiming in!