TL;DR: I counted the rarities of all items on, and analyzed exactly 120 [Prestige III Lvl 50] bloodwebs and 120 [Prestige 0 Lvl 50] bloodwebs for the Spirit killer. The most important thing I discovered from this is that Prestiging has ABSOLUTELY NO effect on Rare (green) and Very Rare (purple) items. You will get EXACTLY the same amount of these for both Prestige types (assuming your character has obtained all possible green and purple perks, as these take the place of a green/purple item that would have spawned instead).

Other than that, these two effects were observed:

• There were very slightly (~2%) more Uncommon (yellow) over Common (brown) items for Prestige III
  
  
• A significant advantage (87% vs. 65% chance of spawning a single red node) was observed for Prestige III for Ultra Rare (red) items
  
  However, both these findings have an alternative explanation unrelated to Prestige, due to an insurmountable problem with the experiment (see section "Caveat" on my Reddit post for more details). Therefore, it is quite possible that Prestiging does not affect item rarity at all.
  
  I repeat: The data show without a shadow of a doubt that Prestiging has ZERO effect on green and purple items, with results unclear only for brown, yellow and red items.

For other facts learned about the bloodweb, screenshots of the 240 bloodwebs analyzed, and all the details about this experiment, please consult the original Reddit thread. Or just check out the spreadsheet[docs.google.com] (though it could be confusing to interpret without the explanations in the thread).

Disclaimer: I am the author of the Reddit thread and the experiment. I counted and analyzed all the bloodwebs, but over half of the screenshots were taken by other people, who are properly thanked for in the post.

If you appreciate the (many many hours of) work that I poured into this project, please rate this guide with a thumbs up! It will really help it be displayed first and become more visible among the thousands of sheetpost "guides" for this game!