I would say that the Xperia P is the smallest brother to Xperia S.
I was also thinking about Xperia P, but I wanted a little bit bigger screen, but with bigger screen, it means bigger phone. It's not very comfy to control the Xperia S with only one hand. I'm a teenager so I don't have big man hands.
And Xperia P was not available in my country at that time, and I desperatly needed a phone, so there was no other choice then Xperia S.
But I think I get used to this big Xperia S phone. It would be even more perfect if it wasn't that tall (tricky to control the phone with only the thumb over the big area of screen).
But if I went for the Xperia P, it would be like my old Xperia X10, with 1GHz processor, 8MP and same screen size, so upgrading wouldnt be that big of a deal, so I just went for something good when I already was buying a new phone in 2012!
Good luck chosing your phone!