Which Tern is suitable for a beginner commuter?
If you can afford gears, buy them. Having an uphill section at the end you will want to shift up in order to make it easier. Going the oposite direction you will also like the ability to go to a smaller gear to get a little more speed downhill. Looking at the pictures linked, it seemed the that the single speed have a rather small rear cog compared to the geared bike. 

If you can't handle shifting rapidly you can always leave it your geared bike in one speed, but at least you'll have the option of chaing it for those morning where you feel weaker or have less energy.