I'm hardly an expert but have a few observations and questions.
I too have read, mostly on this forum, a 17 is a better solo boat. There's quite a few threads about it, and about how finding an affordable, great condition 17 for sale in your area is like finding a unicorn.
From my experience sailing with other people on hobie 18's, in the time it takes me to go from pulling up on shore to driving home (30 minutes max, solo) the 18 sailor(s) are still fiddling to get their boats on a trailer just to put them in lake side storage without even dropping the mast. While I don't know much, it's easy to see an 18 has a lot of running gear compared to a simple 16. How much time do you want to spend getting ready to sail? Without a doubt a bigger cat, with the right crew is more fitting for SF bay, but you have more local options to sail besides just there and how much of a process do you want sailing to be?
Where are you going to store your boat?
Do you want to only be able to sail when you have a crew? Are your "friends" or potential "crew" dedicated sailors? It's not warm most of the time around SF, do your friends really want to be on a small Cat, where you all will certainly get wet and need a wetsuit all the time?
In other words, how much an emphasis of being able to take more than one "guest" should you put on a boat?
No dogs on small catamarans on SF bay, sorry it's not allowed unless your dog is a gold metal surf dog.
How dedicated are you to sailing a Cat in NorCal? You said you've sailed the area before, I'm sure you know then that if you want to sail a Hobie of any size on SF bay then it's not like what your buddy who lived on a Cat is, unless he was camping in a small tent on the trampoline.
I'm not trying to dissuade you by any means, I dove back into sailing just this spring after many years.
Between me and you, hell yes, get a hobie 16, start out in less dangerous waters, get good gear to wear and live a Hobie life! Talk a friend into going out with you every now and then. Anything but a boat I can rig and in often cases upright solo in a short time is not for me. Only you know what you want.
If the questions asked bring you back to a Hobie 16, check out this one, the owner is kind of a dick, but it's a nice boat.
https://bakersfield.craigslist.org/boa/ ... 12696.html