Actually, I think the SSD would be wiser. Swapping to HDD is nightmarishly slow. Swapping to SSD, not nearly as bad! (Remember that much of the slowness of swapping is because of seek times, not throughput. SSDs' throughput is several times that of an HDD, but the seek times are literally a hundred times better on an SSD!!!)
Of course, I absolutely agree that an upgrade to or beyond 8GB RAM should also be done, especially since it's cheap.
But if you can only do one, the SSD will improve performance substantially in all situations, while adding RAM will only prevent slowdowns in RAM starved situations.