They are similar functions in that both can display an expression based search, the main differences between them relate to how they appear in the interface:
Views have these interface features, but not smarlists:
1) Display pane or category views as opposed to just a list of files
2) Can be part of a hierarchy, and can appear as part of the view hierarchy in theater view or Gizmo
Smartlists have these features, but not views:
1) Can be "sent" to playing now, or to a device for sync purposes
2) Expressions that generate large numbers of results (or have to touch lots of files) can sometimes slow down the interface. Putting those kinds of expressions in views can have a worse effect on performance than building a smartlist and only looking at it when you need to see it.
Generally they're just different ways of interacting with and displaying content. I personally use smartlists for maintenance tasks whereas views are more permanent and intended for family consumption. For example, an expression that shows me all the audio files that haven't been analyzed gets put in a smartlist but not a view because it's something no one but me needs to see, and I only look at it once a month. But I could have setup a view for that, they're really very similar.