Mostly the customization that Smugmug offers drew me in. I looked at Zenfolio and a few other sites, but didn't like them as much. 500px is more of a sharing service, like flickr, with the ability to sell images through a stock photo agency. Smugmug allows you to create a unique site, with the pro level account only. Smugmug also allows me to set my own prices, along with the types of prints that are available to customers. Smugmug and a number of those other sites do offer trial periods (Smugmug is 30 Days), so it cannot hurt to try it a little before buying in. If you do end up being interested in Smugmug let me know, I can give a discount code.
Flickr and 500px are a lot less expensive, but they are a lot less flexible. I still have a 500px account, but only a free one, as the community there is far more active than Smugmug.
My
500px vs my
Smugmug website.