heres my 5 cents worth...
ok if yo looking at block and head only yeah they are only slightly different. BUut if you start in the std gtr-rb26 vs gts rb25 the 26 has an airflow meter for each turbo, more piping, more wires.
theres 2 turbos, etc, in the head what vc and rbsx said in the block what rb sx said, then on the inlet side, thers 3 throttle bodies on an rb26 with a good inlet mani from the intercooler ,the 25 has 1 throttle body ,and a nasty intake going across the head to an nasty little intercooler thats way smaller than a 26 has.the injectors are lot different too,not just in size but in type 26 has to feed 25 has side feed( i could be wrong and so on and so on.....
if you gonna spend some cash on mods buy the cheaper rb25 and spend the rest of the money on things that make the rb26 better than the 25,like tubo, injectors,manifold,exhast,fmi, head gasket,and if you gonna go head and block ,cams, flowed head,pistons,rods,etc . my money would be on the RB25. yu can pick up a 25 for R8000 but a 26 you looking at nothig under R15000 if you lucky.i
t would be cool to find a 26 hed for yur 25 .....?!?!?!?!?!? you can always put on a 26 tapet over the 25 head and lie heheheheh,the tapet covers do look a lot better on the 26......?1?1/