I think this particular topic has been discussed quite a bit (try searching the forums). Do note that there are 50% off coupon codes for Smugmug all around the place, so I wouldn't worry about the price difference.
Here is my particular thought, and it really is just my opinion: Zenfolio's themes are much more elegant, clean, and pleasant to use than Smugmug's. I have tested both (I will be setting up shop on Zenfolio within a month), and for me appearance is key: I want my art to be shown as I intend, in a simple and elegant way, without forcing me to write my own code. Smugmug is infinitely customizable, but without editing the HTML and CSS myself, I couldn't make my pages look any better than one of those horrible old geocities free pages. If you're a fair hand with website design, go for it. Otherwise, I'd choose Zenfolio for appearance alone. (I like its other features too -- especially the ability to create coupons.)
Am I wrong but Smugmug seems to have a lot more merchandise too... So if your clients are interested in putting their image on a mug, t shirts, stickers... smug mug seems to have more stuff. I know that's not a reason you'd want to choose one over the other... just an observation... I'm still trying to decide between the two.