Flat bar bikes are great for commuting and the bikes you have posted are just fine for that role. Look around London and tell me what types of bars is everyone else using? I have been to London many times and flat bars reign supreme, there is a reason for this.

With the bikes you posted the Marin Terra Linda SC4 700 is the one I like. It has wide range gearing with the 34 tooth rear cog, disk brakes, and as much as it shouldn't matter, I really like the colour. I would prefer a little wider tire than the 28 mm that it has now. I recommend when the time comes to replace, go with a little wider tire, 32 mm to help absorb some of the road shock.

Fenders are a must even if you ride in on only fair days. The ride home could be miserable and fenders will keep you a little less wet and the bike a little less dirty.

Good luck on whatever you buy.