Both are nice, largish floorstanding speakers, both being somewhat hard to find, if you're looking for mint condition examples, the JBL L-300 will more than likely carry a hefty price tag.
If memory serves me correctly, the JBL 4333 was the Pro Studio version of the L-300, and a prime example may actually be more reasonably priced. The L-300 have been very hot items as of late. No Glass Tops on the JBL 4333, but they are as well a very attractive speaker, with Walnut Veneer, and the Blue Front Baffles yell out "I'm the pro version baby!"
Another highly efficient Speaker, which may be easier to acquire, and I just nabbed me a drop dead gorgeous pair, is usually more reasonably priced, and IMO is a relative killer bargain in today's market, is the JBL 4430 Studio Monitor. Again, Walnut Veneered, 127 lbs each, 35"x22"x16". Made longer than any other Pro Monitor in JBL's lineup, which was 18 years straight, this two way system does not fall short of the earlier 4300 Series family.
It's "Dolly Parton" Horn looks certainly different, and sort of odd compared to more conventional eponential Horn Designs, but this speaker is one that is definitely all business, is seamless in its sound, and is unique, that there are two L-Pad Adjustment Pots, to control Mid, and High Output of the Horn, making it in a way more like a three way system.
JBL did a lot of design, and research on this model, it should not be over-looked as an option.
I've listened to the original Cornwall on more than a few occasions, having a good friend who owns them. While later versions may sound better?, in a showdown between the 4430, and original Cornwall, I'd personally definitely take the 4430 as the speaker of choice.
All speakers mentioned in this thread so far are all great values, meaning they will retain thier value for the future. Of course the most important thing is thier sound, what good is value if you dislike the sound of the speakers? Very doubtful that you would not be blown away by the 4430 as another option for the L-300. Again, IMO, the 4430 is a superior sounding speaker versus the L-300, but does lack the 077 High frequency Tweeter that the L-300 possesses. Mark