Well, I'll take Gallen over any other player if the game is "in the balance" at half time, and you just want the bloke most likely to have a consistent 40 minute impact and lift his team mates.
But, if you are down with 2 to go, hopefully you don't have to rely on him as your go to man as the Sharks have had to all too frequently.
In that situation, if you need a length of the field miracle, Hayne and Marshall are the clutch players I'd want.
On the other hand, if there is 2 to go and you are attacking the line and need the sensible play most likely to score, JT is the one I'd want.
If the siren has gone and you need a field goal slotted over, I'd want Soward.
Geez Lockyer was good in these situations- I'd take him in all those scenarios.
If on the other hand you are up by a point or 2 with a few minutes left, and want to make damn sure the opposition has limited chances to steal the game, I'd want Smith. Also Smith is the man if there was 10 to go and you needed 10 minutes of constructive attack to get back into the game.
Wouldn't be bad to have Gallen, Marshall, Hayne, Thurston, Soward (you'd fit him in somewhere with Marshall) and Smith in your team. I don't think you'd often need to come up with clutch plays though...
If your team made a break or was defending a break I'd want Slater as the support/cover player.