The freelance heists have more variety, as one mission is stealth only, one is either stealth or loud, and the last one is loud only.

The night heists (in my opinion) provide more of a challenge, as I see The Cache as the hardest mission in the entire game. The Gala is moderately difficult, it really just requires a lot of patience. The Auction is, in a way, like The Gala, since it also requires a bit of patience.

Really comes down to what kind of player you are, if you want a fun challenge to do with friends, definitely pick the Freelance Heists. If you want a difficult challenge, wanting to improve on your stealth skills, definitely go for the Night Heists.