Are you planning to sail alone or with crew?
If you have crew and you are not planning to sail with a child you might want to think about a Hobie 16.
Its not a bad first cat,- simple but fast - and you can pick up a second hand boat that's quite cheap. Dart 16 and twix r both quite new so there are fewer 2nd hand boats on the market.
If you want to sail alone than you might want to go for the smaller twix, both the Hobie 16 and Dart 16 would be a handful on your own for a novice