I did tons and tons of research before deciding Cat vs. mono. My reasoning was bouncing back and for for several months but ultimately came down to a couple of safety considerations and a couple of financial ones.
The safety ones were Amel specific; stainless steel lifelines, water-tight compartments, center cockpit, hull-to-deck fusing and the rigging itself. Couldn't find any brands that were built the same as the Amel in those regards.
Financial considerations were resale and docking and fuel. Resale is pretty much always good on the Amel, but I couldn't find a consistently true pattern on the Cats, except at the very high end. Docking almost always costs 1.5x to 2x for a Cat because you take up two slips and with the benefit if two engines comes the fuel expense, increased maintenance and shortened range of running them.
The cats are all over the monohulls for spaciousness and an easy-going ride in easy seas. It's true they are generally faster on the runs and faster under motor. It's also true that (generally) the rigs are taller, the sails are higher off the water, and (as mentioned before) that comfortable rode goes away pretty dramatically in choppy water when one hull strikes a wave differently than the other one.
I guess I also hold the belief that, generically, you cant load a Cat like you can a monohull without the risk overstressing the structure. That's an un-researched opinion, but I believe it to be true.
Of course, I'm going to vote for a monohull because I chose one and I want you to be like me :) Joking aside; we've done 4 rally crossings and he cats all made it just fine. It did seem as though they had some bigger 'adventures' than we did though. It's a tough thing to quantify ... best let a bit of feeling into
the equation as well; ... unless you're planning to sail solo; how does your most important crewmember/Admiral feel about it?
Best of luck! Regardless of how you decide, hope to see you cruising around some day.
SM 209, Adagio