If you want white, lean toward hydrangea arborescens (smooth) or quercifolia (oakleaf). Most arborescens get about 4-5' tall each season but spread. Oakleafs get anywhere from 2' to 12+' so choose a breed to fit your space. If you want some color, you'd lean toward hydrangea macrophylla (bigleaf) - they would die back most years in 5b. But you could try starting with some of the Endless Summer products and you'd get some blooms on new wood each season.

I wouldn't suggest either arborescens or macrophylla in that spot. They would be getting sun during the hottest part of the day, and both would have a bit of a struggle to get enough water to their leaves, especially against a building which reflects light and heat. Usually both are suggested for morning sun, afternoon shade. I would plant one of the H. paniculata varieties which have thicker leaves that deal with sun and heat better. They start with white, cream or lime flowers that after several weeks shift to some shade of pink. When you look at them, check out the shade of pink since some are more attractive than others, look at the flower panicle shape and density, and also be sure to research the size so you find one that fits your space. They grow from about 3' to larger than 12'.

I guess it would help to know where in Zone 5 you are located, deb, in order to suggest a plant that would be hardy and do well facing west. West facing in my hot neck of the woods would be bad for camellias, azaleas and hydrangeas but, depending on where you are, the summer sun might be mild enough in the Summer. But paniculatas are the ones that can probably handle the most sun of all hydrangeas (just not down here).

Also, did you happen to notice if any of the hydrangeas that you saw doing well are facing west or by themselves in full sun? There is a lot to be learned from those who are already growing them in good spots!

I've personally had no trouble with West-facing hydrangeas in Z6 because there's not mathematically enough time for the sun to scorch the blooms much or for the soil to dry out. South of Zone 6 might be an issue. North of zone 6, unless in deep shade for most of the day, 3-4 hours of any direct sunlight be enough for the plant to make food and not get overly distressed.

Droughts and heat waves would usually also pose an issue but are rare and you can fight those by watering as much as any other direction-facing plants and by using mulch.

On this note, slight drooping is normal on hot or sunny days, but should not be evident come the next morning.

Also, smaller or larger companion plants can help by providing additional shade or simply shade the soil. Cooler, moist soil is better for just about any plant.