On the pre '96 there is not much difference between the 2 at all.
All the pre 96 heads have hydraulic valve lifters.
On the 97/98 (v3/v4) the sti's had forged pistons as standard and under bucket shims on the heads/valves, with better cam profiles.
The 97/98 WRX and Uk heads had large above bucket shims.
On the 99/00 the heads were revised again with different castings, cams and inlet manifolds, with low compression pistons and rear thrust cranks.
If buying a second hand complete engine there is no way to tell from the outside whether the engine has forged pistons, unless you had access to a bore scope type camera.
For heads/valves/cams the rocker covers will need to be removed to check.
Reliabilty wise they are all the same as its the big ends that go normally, before pistons/heads on the lower/medium powered cars.
All the cranks, conrods and bearings on the classic engines are the same except for v5/v6 which had rear thrust blocks and cranks, but not really improved reliabilty.
Correct me if I am wrong.
Last edited by Brett_555_Sti; 01 November 2009 at 06:54 PM.
Reason: added info