The better you can communicate your vision for your model railroad, the better the result is likely to be. That's what Armstrong's Givens and Druthers was all about - trying to find out what his client really wanted. Even then, it usually took Armstrong several complete start-overs to come up with the final plan.
Second factor is the space available. For a small layout, say 12 x 12 in HO or less, there just aren't that many options, so a professional designer might not be worth the time and expense. On a larger layout, the designer has more room to be creative and come up with something you might not otherwise think of.
my thoughts, your choices