Answer: You and your husband are not the only ones who get these two shrubs confused. They do indeed have some similarities, and along with a third, Common Snowball Bush, are often mistaken. Let me help 'unconfuse' the situation! Part of the problem is that there are dozens of commonly used varieties of both viburnums and hydrangeas many with similar flowers and habits, and several with round white flowers which are often referred to as snowballs.
The hydrangea that you are probably confusing with the viburnum is Hydrangea arborescens 'Annabelle' which indeed have huge (8" to 12") rounded flower clusters. This is a popular and highly recommended selection with few pest problems. It grows to about 4' tall.
The Chinese Snowball Viburnum that you mention is probably Viburnum plicatum, actually commonly called Japanese Snowball Viburnum. It, too is a nice shrub with white snowball-like flower clusters, but they are much smaller, only 2" to 3" in diameter. The shrub itself can grow to 15' tall.
A third shrub with similar white flower clusters is the Common Snowball, Viburnum opulus var. sterile, "Roseum." This shrub has slightly larger flowers that are 3" to 4" wide, and it's height is somewhere between the other two shrubs growing to about 10' tall.
Any of the three shrubs should tolerate the heat if they are planted in a semi-shaded location, protected from afternoon sun, where they will still bloom. They need moist, but well-drained soil. Water during extremely hot and dry periods, and any of the three should perform well for you.
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