Movements are more or less the same
If the case specs for the speedbird meets what you need and you like the design then why bother changing.
The differences will not just be in brand name, the designs are different so the manufacturing costs will be different, also the Sinn cases are manufactured in Germany which will factor into the costs.
Sinn retail their watches in traditional shops not just online so there are greater supply chain costs to factor in.
Only you can really decide if the sinn is worth more than the speedbird