I own one of each and have sailed many of each plus tris. They are all good, just depends on your intended use and preferences.
Cats are hard to beat for cruising/liveaboard/charter: roomy, stable, shoal draft
, more privacy in cabins at opposite corners of the hulls, better traffic flow...and most modern production cats sail just fine (relative to a cruising mono) regarless of some of the BS you hear. For example, my W35 goes to weather respectably well at about 40d apparent and will pinch to 30d. Listened to mono-biased friends talk trash about cats going to weather one night (neither had any cat sailing experience) at anchor
in the Dry Tortugas
...never said a word. We left the next AM pre dawn bound for Key West
in moderately rough weather and Key West
a tight beat to windward....I had cold beer
waiting for them when they got to KW three hours after me! ;-) Last thing I recall
hearing on the VHF
before they dropped out of range astern was "...where did he go?".
As another post pointed out, cats do tend to have a more "irregular" motion in rough seas, but you don't roll your guts out.
Monos give you more sensation of sailing. My Hobie 33 (a mono) is a blast to daysail, but would not want to cruiser her. Would get to the next anchorage really fast though...capable of almost 20 knots! Sails
in a breath of air and tacks on a dime (neither the strong suit of most cruising cats) and will sail, not pinch, at 28d apparent (per polars and experience)...impressive.
Really like tris for pure go fast fun too. Used to race
on an F-31...break 20 knots with ease!