I am trying to figure out if I should use the 3.5 or PF1e Character Advancement Table for this adventure.
My party of 4 players currently have 6,800 XP. On the 3.5 table, this is 4th level. On the PF1e table, this is 3rd level (and far from the 9,000 XP needed for 4th level).
They are about to descend to the second level of the dungeon.
Any guidance would be appreciated!
That depends a lot on how you do the monsters conversions.
In my case I like to use the Pathfinder version of the monsters, which means a level 4 Pathfinder party has an equivalent challenge to a level 4 D&D 3 party facing the D&D 3 stats of the same monster.
As such, I needed to match the same XP progression in Pathfinder as a D&D party would have. What I ended doing is to follow the 3.5 table and monster experience, and just make them level up at the same point they would have done if they were a 3.5 party.
If instead you are still using the D&D 3 monster stats, I would suggest you use the PF progression so they end up being some level behind, in order to maintain the challenge level.