Gentlemen, some of you are overreacting here. Why avoiding the 4200u like a plague?
Both CPUs are identical pieces of silicon - based on the same architecture, introduced at the same time. The 4300u is clocked slightly higher and has some additional enterprise features that are completely useless if the platform doesn't support them. Which it doesn't. Heck, they are even priced the same - and Intel is well known for asking more for every bit of extra performance or a nice feature
You won't notice an increase of performance in games. The 15W power envelope doesn't allow these chips to fully utilise their CPU and GPU parts simultaneously. The same can be said about heavy, multi-threaded workload. The 4300u might benefit in single-threaded ops, however.
Rarely mentioned is the fact that the 4300u also consumes more energy - as someone already posted, it's around 3W under load. Which means shorter battery life and warmer and/or noisier device.
While I would go for a higher clocked unit in a desktop any day, Surface is a portable device and I do prefer an extra 30-45min of battery life that the 4200u provides, especially when it comes with almost no performance hit...