Cassette & crank combo for commuting
50x12 lets you do about 55km/h (35mph) pedalling at 100rpm. Since you say you're commuting, mostly on the flat, with a non-trivial load, you're not going to get any real use out of a bigger gear than that, so an 11t cog or 52t chain ring won't have any advantage.  However, having a small ring of 39t or a biggest cog of 25t will hurt you if you have any steepish hills (say, 5% or more).

That points towards a compact crankset and either the 12-28 or 12-30. Honestly, there's not much difference between the two for the terrain you describe.  The 12-30 gives you an extra big gear, which is nice if you're going up any big hills but you say you're not. The 12-28 has slightly shorter gaps between the cogs, which makes it slightly more likely that you'll have that perfect gear at any given time.

You might also want to consider secondary uses of the bike.  If you think you might start going for rides at the weekend, the presence or absence of hills on those rides might push you towards the 12-30 or the 12-28.