Personally, I’d ride it fixed; more efficient, faster fitness gains. But if the gear is too high a singlespeed or fixed will feel a chore and you will long for gears (well at least one lower).
How hilly is the commute? 42×16 or 42×17 will get you up some modest inclines. I ride 42×15, and when tired or into a headwind, it is a grind. 42×16 is spinalong fine and geared for 16.7 mph. If you are riding slower than this, then you need to gear down. You could use a three-speed AW rear wheel, for example.
My all-round commuter (and winter road bike) is fixed wheel, (cheap) Kona steel frame with full carbon forks, full SKS mudguards, handbuilt Miche/Open Pro wheels, 9.25 kilos without rack. It flies along, handles like a proper road bike, and is geared for 19 mph (42×15). It is still a chore into a headwind on the morning after a race.
EDIT: for hilly commute; 42×16 = 48×18 is the norm when sold. You might want something a little lower, so perhaps 44×18. You will find 42×18 too low – I bought a used Langster and it had just had a new frewheel – an 18T. It was almost unrideably spinny on flat sections. Fine tuning of gearing is done with the chainwheel not the sprocket.