It's got to be your own decision at the end of the day. In the III's favour are a few extra toys and the extra NCAP safety star. The only thing against it really is that it's so much heavier than its predecessor. If you're running a 1.2 16v now, then you'll want the 1.4 for it to be as nippy.
The rest's subjective. I'm not keen on the interior either, though I don't find the models with climate control that offensive. It's the big sticky-out standard air system controls that I don't like. They're probably great from a useability point of view, but they make the car interior look like it was designed by Fisher Price.
The outside's subjective too, and I still prefer the shape of the old one right now. But then that's my nature - I hate change
The Clio III design will grow on me while, no doubt, the II will start to look dated in time (I don't think it's happened yet though.)
The only thing you can do is take one for a test drive and see if you fall head-over-heels or end up feeling indifferent. If you like it enough to spend the money and you have the cash to spare, then go for it