a 54 Spec is very different from a 57 Onix. The TT differs by 2cm.

The 54 Onix and 54 Specialized are similar. I can't see why they would go up to a 57 on the Onix when the 54 Onix matches the 54 Specialized geometry fairly closely.

Do they stock the 54 Onix also? ask them about it and try them both.