Culver is 6'5 or 6'6". Jackson is 6'8".
And while he has been a horrible finisher of late, Jackson has a far quicker and superior first step to Culver.
IF he can get his head on straight (and that remains a big if), Jackson can fill the slasher role at the 3.
Culver really doesn't have much to offer at the NBA level. His ceiling is a smaller Rondae Hollis-Jefferson (while Jackson's ceiling remains Scottie Pippen or Kawhi Leonard). We need much more than a smaller RHJ.
when JJ adds SIX INCHES to his wingspan, than you can start comparing his ceiling to guys who not only have that on him. Considering that's NEVER going to happen unless Jackson falls in toxic waste and grows into Joshzilla, you can stop using that ceiling argument for him.
Both Scottie and Kawhi's 7'3 wingspan on top of their athleticism is what helped made them be superstars. That length, plus quicks and hops enabled them to guard pretty much anyone 1-4, AND their shot off against anyone, 1-5... neither of which Jackson and 6'9 wingspan and small hands can do.
The ceiling for him right now is more Iggy... and even Iggy had two inches more in his wingspan that made him a
good jack of all trades, but still never a superstar or anything even close to approaching all-timers like Pippen and Leonard.