If I use Tornadus' "Push Down" attack, who chooses which Pokemon to switch?
Your opponent chooses.
Source: Furious Fists FAQ (2014-08-28), TPCi Rules Team (2014-08-28)
If Sawsbuck attacks with "Push Down", who chooses which benched Pokemon to make active?
It says "your opponent switches", so the defending player chooses.
Source: Emerging Powers FAQ (2011-09-01), PUI Rules Team (2011-09-01)