My knowledge of Iberian history is pretty basic, but I know that both countries had what are generally classified as Fascist regimes before, during, and after WWII. I also know that the Nazis and Italians helped Franco to gain control during the Spanish Civil War. Why didn’t Franco return the favor during WWII? And would the entry of Spain and/or Portugal on the side of the Axis have made any kind of material difference to the outcome of the war? At the very least it would have forced the Allies to open another front, right?
A very similar question came up a few weeks ago - I wrote a bit on the Spanish perspective, while u/Abrytan discussed Portugal. The short version is that the equation never quite balanced - Spain required a great deal in terms of support/rewards if joining the war was going to be worthwhile, and from the German perspective the price Spain required was always a bit too high for the perceived advantages they would gain from them joining the war.
As for whether it would have made a significant difference, that's inherently speculative. One big difference is that it's unlikely that the Franco regime would have survived losing the war, so I suppose from a Spanish perspective that is one rather large change to the war's outcome. In terms of the course of the war itself though, I'm not really sure how to give a good answer. Gibraltar would have likely fallen, but I think the Allies were prepared for that contingency. Spain was vulnerable to Allied naval and economic power, and was exhausted after years of war, so may well have been a drain on German resources as an ally. It would have extended the Atlantic coastline that needed to be defended against invasion. So, while I don't think Germany would have accrued a decisive strategic advantage from having Spain in the war, it's impossible to say how it would have affected how events unfolded.