Reflective cap sheet vs aluminum oxide coating

I’m considering two proposals for a roof replacement. This is an 800 square foot low-slope roof on a row house.

I believe both bidders are reputable. (Licensed, insured, been around a while, good reviews, BBB, etc.)

Both bidders propose modified bitumen and both propose reflective roofs. But for the reflective surface, one proposes a Certainteed white membrane cap sheet and one proposes an aluminum oxide coating instead. Since the latter has to be re-coated periodically, I tend to favor the white cap sheet. Am I right in thinking that the cap sheet is cheaper/easier to maintain vs the coating?

The bidder that proposed the aluminum oxide coating came in about 20% lower, which is substantial but will not be the main determining factor.

The bidder that came in higher was more specific about materials in their proposal. They use Certainteed materials, including 1.5" insulation board, SBS base sheet, STA perimeter membrane, GTA Coolstar cap sheet.

The lower bidder was not as clear about materials in their written proposal. Their proposal referred to the roof membrane as “torched down rubber roof membrane.” When I called to ask about the membrane they clarified that it’s modified bitumen and not EPDM rubber. They’re certainly happy to answer my questions and I don’t think they mean to mislead. But their written proposal is just not quite as sharp as the higher bidder’s.

The lower bidder proposed 0.5" insulation board, and suggested that there is not much insulation benefit by going thicker than that, since the board has an R-Value of about 1.5 per half inch. They said they would be able to use a thicker board for a few hundred dollars more if I want.

So, the main differences are the white cap sheet vs the coating, the more detailed proposal offered by the higher bidder, and the thickness of the insulation board that each initially proposed.

In addition to these bids, I received a couple other bids that I’ve ruled out.

Thoughts? Thank you!