If we're talking about the Good Friday Agreement, I think nationalists probably got more from it, but that's only because the nationalist community were effectively second-class citizens beforehand. Unionists still maintained a lot of dominance in Northern Ireland, but they had to cede total control of a (gerrymandered) parliament to a mandatory coalition between the largest nationalist and unionist parties and the renaming of institutions like the RUC into the PSNI. Both sides had to agree that Northern Ireland's constitutional status could change, but only via referendum. The IRA had to disarm and disband (although splinter groups still exist), while no loyalist paramilitaries had to do the same. Hardly anyone from the British army or the RUC was charged for the collusion or the extrajudicial killing of civilians. You can tell who felt they got the better deal by the fact that nationalists overwhelmingly voted in favour of the deal, while unionists were more divided.
If it's the Anglo-Irish treaty that created the Irish Free State, it was likely the British that got the better deal. The treaty was controversial and led to a civil war in Ireland over its terms, it meant Ireland lost 6 of its 32 counties which became Northern Ireland, and the country wasn't allowed to become a republic like they had wanted.