On my Galaxy Note I used to have only Smemo, but when I upgraded the Android OS I saw that Snote had arrived, which seemed so alike I wondered what the advantage was.
You may be able to find various differences but this is the big one: Smemo will sync with Evernote. By that I mean if you log in to your Evernote account you will see the memos you have created just as they appear on your phone, images and all.
Snote says it will sync with Evernote but all it will do is save them as files that Evernote can not read. Log in to your Evernote account and you will only see the Snotes as attachments that can be read in your browser.
If you want something to take notes with, using your Spen, I have found Papyrus to be handy. It is like having a huge piece of paper, to draw and write on, that you can zoom in and out of.