Shimano or Microshift derailleur
 

Hi there was wondering which brand would be better Shimano or microshift? And its an 8spd cassette long cage.

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.

^This^

I just replaced a front, 3 spd Shimano front road derailleur which was an old model prone to breakage, with a Microshift front derailleur. The Microshift works very well, I would not hesitate to purchase one again. Appears to be well made. Of course, with the temps here of late, my wife hasn't put miles on her bike. I know you're talking rear der., but if it's made as well, should be fine.

I would rather have gotten an older 8 speed LX or XT front derailleur shimano from ebay much better than microshift.