I tried on a half round and a flat 2mm plat band, and although they were exactly the same widths, the flat band looked wider. I have a mostly flat e-ring and am getting a half-round and comfort-fit (rounded inside) w-band. I think either way will be fine.
I have a girlfriend whose e-ring is flat inside, and the w-band is comfort-fit (rounded inside), and she doesn''t have any problems with it. Try some bands on and see what looks best and feels best to you.
I have a 2mm 14K HR, a 3mm Plat HR, and a 3mm 14K Flat band. Of the three, the 2mm 14K HR is my favorite.
I think the HRs look better with my e-ring. The 3mm Flat is too noticeable when next to e-ring.