Hard to answer as it depends what's more important to you with the phone. A cold choice between the two, it'd definitely be the Siii with no hesitation whatsoever.
It's a great smartphone and a great version of android. The experia is way behind in terms of build quality and use.
However, if you really just want a phone as a phone with small use of photos, social media, etc, then with the premium Siii, you're paying for something you're not going to use to its full capabilities.
If you don't mind spending the €150, go with the Siii. But if that's a bit expensive and you don't think you'd really use it as a smartphone (though a lot of people say that until they get one), then you should be ok with the experia.