My 2.0 Celica GT has done 70k miles and I am considering applying an engine decoker as I have heard thet this will improve performance - does anyone know if they are worth the money? Apparently they can improve engine performance by up to 15 BHP in some cases.

I have 2 choices, a gas based one that goes directly into the air intake (e.g. Ecotec Powerboost or 10K Boost) or a liquid one that goes into the fuel tank (e.g. Lucas Injector Cleaner or Redex). Which one is best?

That wont gain you any performance, but will improve any lost HP you have on the worn engine. I'd go with the injector cleaner in the gas tank. Thats what i always use.

yea but injector cleaner won't do what he wants to do,
that will only clean the injectors where as the lucas type and others will help restore the cylinder walls and rings and such

I've put in half a bottle of Lucas cleaner into a full tank. I'll let you know how it turns out.

I put a full bottle of the Lucas Fuel Cleaner last oil change and it seemed to smooth out the way my car ran. The key word is seemed, I'm not saying you'll feel a difference also, but for $3.50 it couldn't hurt.

I'm at 210000kms so I think that I might try both...
Injector cleaner in the gas tank first, leave it until the next fill up and then the powerboost in the intake.