OK FOOLS (language moderated for Chris's sake)
Seeings as this damn question keeps coming up, why not have ba bit of a sticky to prevent yet another pointless discussion, reeking of bullshit and falsehoods.
1. The engine is exactly the same engine, same model, same management. The engine itself runs the samemap and I'm pretty sure there isn't anything in the Magneti Marelli ECU to tell it if its in a GTI or a VTS. It simply doesn't care. The ECU does however have different software versions on it depending on the age of the car. The VTS does have slightly different cooling, it has a heat exchanger to exchange heat between the water and then oil. This makes sod all difference to anything
2. The Gearbox is the same. All the ratios are the same, as is the final drive. Everything about the gearbox is the same, excluding the gearnob, which I think may be different on later GTIs.
3. The suspension is DIFFERENT. but not by much. The ARB is 24mm thick, compare to 22/19.5mm on the Saxo. The damping rate and spring rate is also a touch different, though this is largely irrelivent as most GTI's/VTS's run "uprated" springs and dampers, which are mostly selected by brand name and the amount they "slam" the car rather than by unimportnant details like spring and damper rates. The reason for the different setups is that GTIs come on 14" rims as standard and the VTS has 15" from 1999 as standard
4. GTIs have leather from 98 onwards. VTS's don't. From Marthish 2001 they both had airbags. Both seats are pretty similar, though I find my 2003 leather efforts give a bit more support than my 2003 VTS seats, probably as the leather is a bit tougher than the fabric.
5. The VTS actually cost more than the GTI new according to the price lists, however, the VTS always got more discount. Due to Citroen's free insurence/cashback/barter approach my 2001 VTS was bought using unwanted children, stamps and a really nice victorian 3 piece suite. The discount got a bit mental from around 2001 onwards which is why you don't see many 106 GTIs about from after then, they would have cost a couple of grand more overall.
Weight - God knows. Both companies have different policies on judging the curb weight of their cars. I'm guessing they're sod all difference howver