Hi Alex,
In regards to the TeraFlex Dana 30/Dana 44 JK HD Ball Joint Set Without Knurl here is what TeraFlex says about the differences:
We recommend Ball Joints w/out Knurls (# 3442002 or # 3440000) as they will not score the inner knuckle & open up the bore
We also offer Ball Joint w/ Knurls (# 3442022 or # 3440020) for ball joints that have a loose press fit in the inner knuckle due to repeated ball joint replacement or if knurled ball joints have been installed in the past.
So I read this as if the bore is not damaged and or you have not replaced ball joints before, keep without knurl. If the bore is a little loose, use knurl.
If you have any further questions you can contact our customer service center @ 877-474-4821
Hope this helps