STI struts are the ones capable of supporting stiffer springs but the damper rates on there leave much to be desired. You'll end up with bouncy rears, which you can mitigate with stiffer top hats (e.g. Group Ns). GR2s are stock replacements, said to be only slightly stiffer, but the feel is almost negligible to most. Also, if you go with STI struts, you will to use 04 STI fronts and 04-07 STI rears. STI struts can also be notoriously clunky if you don't repack the grease before installation (inverted struts).
I would imagine the sti struts are more aggressive.
I'm not sure for the foresters but for my 02 wrx, the gr-2's advertised only 10% stiffer then stock. I've been pleased with the gr-2's and probably will use them for the wife/boss FXT when the stockers go.