George's comments are right on with the differences. Both boats ride and drive great, they're just fundamentally different. I love my 18, but I've always wanted a Minx. I understand that I'll have to either take on a second boat some day, or make a trade-off of the fun/excitement factor for the comfortability factor.
Like trip said, a lot also depends on your passengers. If I had a longer boat with a windshield and higher freeboard (Minx), I would have more people prone to ride with me. Good thing I only like hanging out with exciting people.
Good luck with the search. Try to find a couple to hop in, or at least test drive whichever boat you lean toward before making the purchase.
1970 18 Classic-original
"Beer is living proof that God loves us and wants us to be happy." -Benjamin Franklin