Both are nice solutions, I have used them both and ended up going with Zenfolio. Both have awesome customer service, both offer a good service and both are constantly changing with the times, which is always a good thing.
One thing I disliked about SmugMug was it's admin control pannel, it's very archaic compared to Zenfolio. They both do the exact same thing, Zenfolio just does it easier. Zenfolio is more GUI(esk) which Smugmug is more drop down boxes than I ever want to deal with.
Zen has more vendor partners integrated into the system, which is awesome. Bay Photo is an amazing lab, but they don't offer the same amount of products as Mpix. Not to mention Zenfolio has more than just one lab to choose from.
I also went through and did a test order with both sites and found that the Zenfolio ordering system is a lot easier for the customer then Smugmug. I also get told this by our customers pretty often, one of our competitors uses Smugmug.
Smugmug is the leader when it comes to customization though, but with that, I always come across some horribly designed Smugmug pages that are a pain to use. They are slow and no where near user friendly, but that is not Smugmug's fault at all, it's the owner of the site. But with that, if you have many horribly designed sites that are pulling loads of info from the server, it will slow your site down. Which is why I like Zenfolio's lack of customizable features. A controlled environment, especially when it comes to web servers, is always a good thing.
I like Zenfolio for its easy Admin interface and its larger selection of Vendors. I like Smugmug because of its amazing customizable options. As with any service, each have their strengths and weaknesses, give both of them a trial, they both offer that option. When you do a trial at Smugmug you get the Pro account, I know that much.