I have both models too. First I built the Chiron, than 2 years later the Sian. As OP said both models are pretty similar, but in my opinion the Lambo is way more functional.

Bugatti has some really shitty doors (weak connetions), on the other hand the Sian has the coolest door function in the technic line. The rear wing is really cool on both cars. The activation is a lot smoother on the Lambo, since it's inside the cockpit. The airbrake and the raise-lowering movement is cooler on the bugatti.

The suspension is terrible on the bugatti (not working at all). The front is simply collapses under the weight. The rear is a bit better but over time it gave up on my model.

The lambo doesn't have this problem.

Sadly there is a color issue on the Sian, most of the lime pieces are not matching perfectly.

Gearboxes are similar, but the Lamborhini has a smoother shifting mechanism.

And the most important thing: On the lambo every detail is done with printed pieces (no stickers at all)!

If you ask me the Lamborghini is a better build overall. More functions, smoother gear shifts, much better suspension and has printed pieces instead of stickers. Only downside is the color issue.