I'm not a professional tarot reader, but -- if I were -- a major objective would be to have my clients return for further readings, and tell their friends (in a good way). To that end, I'd wish to use a tarot pack that looked impressive... and probably one short on images that look too scary.
One reason I wouldn't use the RWS is that some of the cards look quite scary when they shouldn't. I think especially of the Seven of Cups which (judged solely on the image) has always looked to me as though it should mean something very bad. Another reason I wouldn't use it is that it doesn't look (or no longer looks) sufficiently impressive. It's imagery has grown overly familiar (used, for example, in the "Bitter Suite" episode of "Xena").
The Crowley/Harris pack has the advantage of less ubiquitous imagery -- and, I think, looks suitably impressive. I may be forgetting something important, but I don't recall it containing any images that would frighten clients overmuch.
I don't think I'd use the Marseille tarot... or, if I did, only use the majors. The cards from ace to ten might (from their appearance) mean almost anything. Clients might think that I was making it up as I went along... Or, at least, that's what I'd expect clients to think.
I think that, were I professional reader, I might opt for one of Kat Black's tarots: either the Touchstone or the Golden.