I love my 18! More buoyancy for an occasional 3rd or 4rth person to sail, and the bows have so much buoyancy that you very rarely have to worry about pitchpoling. On a bigger lake that is known to get rough, I wouldn't hesitate to take an 18 out, but a 16 would make me nervous - too easy to stuff the bows. I've been on Lake Erie in 4-6' waves and my 18 handles it better than my family's 27' Grady White! Plus the expandability for wings, the in hull storage, and the increased tunability of the rig makes for an overall more accommodating and faster boat to sail. I raced in the madcatter as one of two 18's, and I was overtaking 16's despite their 5 minute head start! And I'm no expert, either
I would say that the 18 does point higher than the 16, though not drastically. There are more options to power up or de-power the sails, depending on your needs. The 18 is more challenging to right, and much more time consuming to rig and de-rig. From strapped down on the trailer to sailing away from the dock, fastest I've managed was 45 minutes, and it's even longer to tear down at the end of the day (or so it seems). I think it really comes down to what are you looking to get out of it? I think the 16 and the 18 are pretty comparable in the "fun" column, so consider that to be a wash. 18's are heavy to get around the beach, and a bit more work to set up, but you kinda make up for it in tunability and versatility. The mast is a PITA to step thanks to the diamond wires, but two people can make quick work of it. Like I said before, the 18 was MADE for rough water, the 16 is perhaps a little less suited towards it. Most 16 sailors I've talked to complain/worry about the daggerboards, and I honestly don't understand it. It takes me all of 10 seconds to raise the daggerboards when I need to, they're not that hard to cope with, and they allow the boat to have a faster symmetrical hull form, tack better and point higher. Seems like a no-brainer.
All that said, after 3-4 years with my 18, the constant setup & teardown is starting to be a drain, and has left me without a dedicated crew, with less sailing time this last year as an unfortunate consequence.