Nikko Hydrangea blooms on old growth only. Which means the year prior it sets its buds. So if you get a bad winter or late Spring frost or freeze it will not bloom. Endless Summer Hydrangea blooms on new or old growth and also reblooms every season up to the first frost. So no matter what every year you are guaranteed to have flowers. ES look very similar to a Nikko. You can get them in my area at about every local nursery, Walmart and Home Depot.

If you want size and can protect, Nikko is a safer bet for 2 reasons.

1. They grow 2' taller than ES. You can have a 6' tall shrub year after year.
2. They are somewhat less likely to be killed to the ground by late frosts - I've seen this for myself.

When ES is killed to the ground, it can grow 2-3' in one season, and will repeat the next, so 2-3' may be all you get out of it. Nikko's should be fine in 6b with very light protection. Add aluminum sulfate and you're off...