/ProfessionalWebDesign/ - ambiguous words (webdesign, web design, sign)
The spider reads and tries to guess where the words are in the phrase.
possible words (from the top of my head) in professionalwebdesign: pro, profess, sion, ion, web, sign, design, etc.

/Professional-Web-Design/ - no ambiguity, words are separated
This is better. You are showing the spider where the words start and end, but it's still not perfect because you have no specified which are hyphenated words.
ie.: the phrase "professional-web-design" could have 3 words "professional and web and design" or 2 words "professional and web-design"

/Professional+Web+Design/ - no ambiguity, words separated with URL equivalent of space.
This is a perfect match. That is because you clearly separate words and they won't get mixed up with hyphenated words.