I go to UCL so obviously I'm biased but I would absolutely pick it over Kings. I know a number of people studying chemistry and I don't think any of them actually specifically want to pursue it afterwards. Athough they enjoy the subject, part of the reason they chose it is because it is such a transferrable degree that endowes you with a broad skill set. So if you picked UCL you would still have lots of options after graduation.
There is also a fair amount of rivalry between the different London Unis and although it is friendly, KCL get so much stick for being the underdog and are commonly known as 'the Strand Poly'. I had the choice between UCL and Kings too and I know it sounds petty, but at times when everyone is making fun of Kings, I'm so glad I go to UCL! Do you really want to be burdened with that image?! Forever regretting your decision not to pick UCL? No. Well then you have your answer.
Jokes aside though, it really is amazing, I love UCL. Just make sure your decision is based on your genuine happiness, (cliched as that sounds) and not image.