I have both and it is like comparing apples to oranges. Both are wonderful toys made to last for years. The Eroscillator feels very similiar to a regular vibrating toy, almost to the point that it is hard to tell it is not just vibrating, but it does not cause any numbness "down there" like vibrators can and the resulting orgasms are extremely strong. (*by numbness I'm not talking about a permanent situation). The con would be that it is probably not the best choice for couples sex.
SaSi is unlike any other toy on the market and she simulates oral sex very well, as well as has a vibrating feature, you can read more about it in my review link
I don't know that she would be the best choice for actual sex either, but is a wonderful foreplay companion, and also provides stronger orgasms.
Now, I'm sure everyone has a different opinion but bottom line is, I like my SaSi better.