If your winds are consistently over 15 knots, you would be better served by having a smaller genoa, say 110. You'll be really unhappy in a 20 knot wind if your only choice is a 150, because then you'll either be fighting the helm constantly, or taking the headsail down (as some would suggest). Then, you'll find that your masthead rig really sails like a dog without a genoa.
If you get bored easily in light winds with a 110, then get a larger sail to back up your 1st choice. If you are hanking on your headsails, you'll come to the conclusion that you want a choice of at least 2 sails pretty quickly. The headsail is your engine for a masthead rig. You'll be happier if you have a few choices to match the conditions. If you have a lot of wind, as you suggest, I would also add a backstay adjuster so that you can keep the forestay appropriately tensioned for upwind sailing. A loose luff in a high wind will create enough sag to make your sail shape and boat handling miserable.