Hi,
A Hobie 17 won't give you much more cargo and passenger capacity than your 14. It's designed as a solo boat. A Hobie 18 is do-able with one person, but with dagger boards and all, it's probably a handful. A Prindle 18 (my boat) is simpler (and a little slower), but makes a great solo boat that can easily haul a passenger or two. Either way, you'll need to invest in a righting pole or Solo~right to recover from a solo capsize (and don't think it won't ever happen). If you are thinking about the possibility of a spinnaker, and you have the cash, you might consider one of the newer "high tech" 18 footers. I'll let someone with experience comment about them.
When I switched from a Hobie 16 to my Prindle 18, I was shocked at how much nicer the boat handles weight, waves, etc. I'm not saying it's faster, but it's so much nicer to sail if you aren't into the racing scene.
Of course, the final decision will probably be how most of us decided on a boat......You will either stumble on a deal, see one for sale along the road at a dealer, or someone in your area will tell you they saw one parked behind a barn for the last couple years??

Good luck,
Jack