First of all let me say that going from 512 MB to 1024 MB is a must when you want to do some serious work. Windows feels a lot snappier, especially when working with large documents or many applications. Why would you buy a notebook with a high-end CPU only to limit it by available memory.
A big surprise for me was that the gains from running two modules in Dual-Channel are very small, barely measurable, sure not noticeable during regular work. This is why I would suggest getting one memory module of 1 GB over getting two modules and ending up with one extra module which you can't use. Also this gives you an easier upgrade path in case you need even more memory. The more memory you have the higher the power consumption of your notebook, so it may not make sense to install extra memory you will never need.
During all testing and day to day usage I never saw any issues with the G.Skill F2-4200PHU1-512SA. They are stable at all times, even when the notebook bottom is warm due to heavy usage.
The most exciting "feature" in my opinion is the price. Of course if you buy an expensive notebook you probably have the cash to buy expensive add-on memory, but why should you do such a thing. There are no additional gains, actually the G.SKILL memory comes with better memory timings than the Lenovo memory and you save a couple of bucks which you could invest into a nice notebook bag. I don't see much reason to spend more money on buying memory which can be clocked faster, the gains are rather slim. If it's only a few bucks, sure go for it for the better resale value.