I've been sailing for quite some time, and I'm now looking into buying a boat.
I usually rent the Hobie Getaways, but I've also used the Prindle 18'.
My question is which of the following would be the best to purchase
(i.e. best quality boat...least likely to break and the most fun to
sail):

Hobie Cat 16'
Sol Cat 18'
Prindle 16'
Nacra 5.2

If they are all relatively equal, what are the pros and cons to each?

I'd recommend a 16 foot boat if you're going from rental Getaways to
owning your first cat.  The Hobie 16 is an extremly popular and common
choice.  For that reson they are very easy to find (new and used). 
Parts are readily available and they have a great track record.

The trampoline is raised off the deck which keeps you a bit more dry
than some of the other beach-catamarans.  They have optional trapeze
wires which really makes sailing them exciting.

Because they're so popular, there are fleets of Hobie 16's all over so
if you decide to race it, you're likely to be around other Hobie 16
owners.

The other boats you're considering are simply less common.  Especially
the Sol Cat 18.  Prindles and Nacras are certainly not uncommon, but
they just aren't as popular as the Hobie 16 and for that reason, it
may be slightly more difficult to find boats for sale, parts, and
others in your area with the same boat.

Hope that helps!  Good luck!