The Karma is a very different boat.
The main question is: Do you like your Zen? Is it comfortable? Can you make it do what you want it to do? Is it easy to roll? If the answer to those is all yes, there's no reason to change. The advantage of the Zen is that you have lots of extra bouyancy for, say, overnight gear if you get that urge, or plenty of room for winter gear and such. The Karma will be subtly different, due to the chines and hull shape. It's quicker, but won't land as softly on a big drop, and is more vulnerable to being driven off course by shallow rocks.
A lot of people like to be at the bottom of the weight range for creek boats ecause of the extra bouyanncy. There's no magic formula. You should think of the numbers as guidelines. For instance, people with long legs and big feet tend to size up for the additional comfort for their feet and knees. Slicy boat padddlers tend to size down where possible to improve tricks., and we won't discuss squirt boating. That means that, at this point, you should try as many boats as possible to figure out what you want in a boat. Just don't sink a lot of money into the process. Try your friends's boats at lunch stops, put ins, takeouts if you have the energy. If you think you're happy with the Zen, there's no reason to change.