Hello reader, with this guide I will hope to inform you clearly on the XP each heist can actually give.
All tests were performed on a save at 0-70 that was reloaded after each heist, there were no bonusses, the stealth heists were all solo-ed, the loud heists were mostly played with pubs.
Sometimes methods like going without extra loot are not placed, that because the placed methods are much better with XP without being much harder.
Even though RNG can be a big influencer on your playthroughs the differences between heists are still big and different RNG wouldn't change their position a lot.
More maps, difficulties and options over time.

Remember that playing a heist again and again will give you a XP penalty.
You could also just join someone elses high crime spree but that's exploiting Overkill's inability to balance that.
I'm not using speedrun tactics/exploits as I want the listed times to be realistic for the average playthrough

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Disclaimer: Not all heists will be listed as it's much easier to stealth them.
The general difference between Overkill and Death Sentence isn't very large and Overkill is also easier, unless you want a challenge Overkill might be the best option so you won't lose XP time by failing/going down, etc.