It's not an issue of advantage for which form to use. It's an issue of Facts and Circumstances.

Sched C = they are going to be doing this as a business or side business.

Sched D = they did it once, such as bought a house for themselves, but got an offer and sold it instead of keeping it = investment.

And what you have is not Expenses, These costs are part of basis = invested in the property sold.

If you have ongoing operational expenses (not directly related to what they bought, improved, and sold), they likely are running a business = Sched C.