You could argue both classes are similiar because they're both relatively privileged by their position within capitalism and thus incentivized to protect the status quo. But their relationship to production is different.

Petit bourg are small business owners, independent contractors, self-produced artists, etc. People who technically own the means of production but don't exploit much of others' labor to extract profit.

Labor aristocracy is a looser term. It refers to a section of the working class (people who own no capital and sell their time and labor for a wage) that benefits from the exploitation of other sections of the working class. Some people take this to mean all workers in the imperial core, others use it to refer to supervisors and managers and the like, and some use it as a stand-in for the PMC - doctors and lawyers and such.