Both structures you were quoted are real and in use. There is no single standard for lease-up PM compensation, and that is the actual answer to your first question.
The 8 percent of collected rents model has an obvious flaw for new construction: it pays the PM almost nothing for the first 60 days when showing volume is highest and their labor is greatest. Good firms know this, which is why you see blended structures or the flat monthly fee instead. The $4,500 flat until 93 percent occupancy is a more honest reflection of actual work during that phase, since they are running showings, screening applicants, and executing leases before significant rent rolls exist. The question is whether $4,500 covers their cost or whether they are treating it as a loss leader, and that distinction matters for what you get.
The assumption doing the most work in your current conversations is that lease-up and stabilized management are the same service with different pricing. They are not. Lease-up is sales-heavy and labor-intensive. Stabilized management is administrative. A firm that prices them the same is telling you something about how they think about the work.
The risk you have not named: PM firms that take lease-up at a loss to win the long-term contract have an incentive to call the asset stabilized before it actually is. 93 percent sounds like a clean threshold, but on 34 units that is 31.6 units, so you need to define in the contract whether "stabilized" means signed leases or paying tenants at 93 percent, and whether any single-month dip below that resets the clock. The language in the management agreement matters more than the fee structure.
Your contractor is right about the 90-day lead time, but I would also confirm whether any of these firms have handled new construction lease-up specifically in the Durham submarket, not just stabilized acquisitions. The submarket dynamics affect their ability to price and execute.
What does your pro forma assume for lease-up velocity, and have you modeled the carrying cost difference between those two fee structures against different absorption timelines?
