There is a differance between the two, but not a great deal, so unless you are in masters and need more grunt, then let your wallet decide.
I personally think the 170 can be a bit more user unfriendly during breakin, but after that there is no problem.

But with saying that, there are two ways to upgrade or modify your 160 to about the same as a 170, one of which my son, Adam, used in the Worlds at Argentina,
and the modified 160 was used in preferance to his 170.

If you are going to spend $500 on a s/hand std 160, I will sell you a upgraded 160 for the same price, the upgrade will give you about 450 rpm increase on your setup.

It does not really matter if the engine has only twenty flights from new, or has been rebuilt after 2000 flights, you will get the same performance from both of them.
Adam's modified 160 started out as a very tired engine, I did the mods and was impressed with it performance, so then I replaced the piston, liner, ring, bearins etc,
it is now a very nice engine with possibly better grunt than the 170 down low where you need it.

Will Crossman
YS Service Aust