Both are going to be pretty equally low end at the 8 speed range. The only way I would say one 8 speed is marginally better is if you were swapping to Microshift Acolyte but that is modern groupset and not just an S group 8 speed so it is compatible with it self as it uses different cable pull ratios.

The only reason to swap a derailleur on a new bike with 6-8 speeds would be to replace it because the old one is not usable at all anymore. Generally in that case I would use the same derailleur I had unless for some odd reason I could get an upgrade for less then the same derailleur or somehow they gave you a nice frame and nice suspension and other parts but really cheaped out on the groupset which is rare to non-existent.