This thread is akin to being asked which testicle do you want removing. Both have given me many happy memories but it's still possible to function with just one of them.
I'd hate to chose but if I really had to, I would keep SIlva. Purely because if they both lost a yard of pace, Silva would still be capable of playing the same way and would still be a world class player. If Aguero lost a yard he would be nowhere near as effective.
It's a strange one really, because if we put them both on the transfer list today, Aguero would command a much higher fee, but he is also the one who is easier to replace. Although you wouldn't be able to find a like for like replacement for either of them.
If either of them got an offer they couldn't refuse, I wouldn't begrudge them a move. Both of them have repaid their transfer fees twice over and then some. I would be distraught at the thought of either of them leaving but I would wish them well. Unless they went to the rags of course.