Dorman vs GM rear main cover kit

[hikerboy]

Looking to get my '05 Sierra 5.3 rear main seal and plate gasket replaced...I see the Dorman 635-518 kit says "improved design", and the GM one 12639250) says "newer design", and it has 13mm head bolts instead of the original 10mm size. My mechanic says he's used Dorman without issues but the GM unit might be a better option in my mind? Thoughts or experiences? Thanks!

[davester]

If it bolts in place, is about the same shape and the oil fill port is at the same height, then either is good.