Humm - Hard to say what is the best choice. Really comes down to preference. White Moth is one of the earliest to bloom and does bloom for a long time. It looks like it and my Preacox are about to bloom at the same time (first time Preacox has bloomed). It has a nice rounded shrubby shape. Here is a picture at the beginning of it's bloom time last year:

I would leave the blooms. The Limelight is very hardy. Just give it plenty of water the rest of this year. Next year it should not require any extra water. Also when you are planting before you back fill the hole. Fill it full of water and let it soak in, then back fill in around it. Along with digging the holes atleast twice as large as the rootball. Also hard prune them after blooming in late fall through early spring before new growth begins in spring. The harder you prune them the larger the blooms you will receive. Good Luck!

Delta Snow does better for me than David especially when watered "excessively". They are in an island in the center of the lawn and get some water for the lawn sprinklers. Half the stems of David and another (Blue Boy? - not sure) turned black and died. Delta Snow was fine.

What size was it when you bought it? Sometimes, particularly if they are in small pots when you buy them it takes a few seasons to bloom properly. The sun should not be an issue as four hours should be enough to get it to bloom. Paniculata hydrangeas (like Bobo) are reliable bloomers so they only think I can think of is it needs more time. Also, I would apply lightly a granular time release fertilizer in early spring.