i use zenfolio. never used smugmug.
i love it because it's easy to create a folder hierarchy and organization pretty much in any way you want. if you go to my website (pics.xyclopx.com
) you can see it's organized very much like a directory structure on your own harddrive, and that's on purpose. most other people do it differently though, and again, it's so flexible can do it any way you want, and name the URLs pretty much any way you want. that sort of organization is impossible on any other consumer-level picture hosting service like flickr, 500px, etc.
seo-wise, it's hard to say cause much of it has to do with the user, in addition to the code. but i would just guess based on my web experience that they might be behind others in SEO. my images simply do not show up in searches even with tons of keywords and good descriptions. the website itself does, but not the subject matter. again, that may be mostly my fault, but not sure.
anyway, i'm happy with it for the most part. my only real complaint is that very recently, like starting about 1 month ago they started having performance issues. these problems never occurred in the past. the ceo even sent out an email to all users explaining their problems. so they know of them. anyway, it's better now but not completely cured.