Now that the whole market is GI, and the metal tiers are the same in small group as in IFP, there won't be much difference in price. That is why many folks think the implosion of the small group market is coming.
In the SHOP exchange, a small group with low income workers might qualify for a business tax credit, which would be one reason to keep a group plan.
The small group plans might have better networks than exchange IFP plans. The network differences are yet to be announced, though.
If the staff is mostly low-income, the premium subsidies for those under 400% of FPL and CSR (cost sharing subsidies) for those under 250% of FPL make it more attractive to employers to discontinue paying for group coverage when taxpayer funding is just a step away!