Most likely Spain pulls a Turkey, but it's plausible that after the Allies take Paris that Spain might join. But Spain didn't have much to offer. Spain had been wrecked by the civil war and wasn't in fighting condition, all they could offer were things the Allies already had, some ships, some troops, some airfields, some ports, etc. So Spain's impact was not going to be meaningful in anyway and the Allies would know this, they are only going to accept Spanish help to make sure Spanish help to Germany stops. Anyways, what is going to be interesting is Spain's role in the Cold War. Though they wouldn't have made much of a difference in the war,politically Spain will be in a completely different position. This probably leads to some boons of sorts, but don't expect anything like Security Council.