wot will be a better turbo to put on my sti its got a td04 at the mo

Wotcha and welcome -

 

It depends what else you have done to your car. Compare blowing through a hose pipe against blowing through a straw bolting on a bigger turbo will not necessarily give any gains but will generate more heat and back pressure resistance.

 

TD04 is the first part of the code for the turbo and is the same for certain Legacys and Forresters but the internals are different giving different power delivery characteristics.

 

Not knowing any further details at this point I would suggest retaining the TD04 size and look at a unit which will spool up quicker using a billet bearing or with a different blade profile dependant on how you would like the power to be delivered.

 

Here are some options for bigger units that cover your year but it is not as simple as bolting on an upgrade

 

http://www.ra-motorsport.co.uk/Product/Category/NA20100917115521