There is a slight advantage of an SSD to a HDD, but there something to understand first.
An SSD is faster than a traditional magnetic drive because it is able to read/write faster. Due to the architecture of the PS4 you will experience things like the operating system running smoothly.
But, in terms of games - the data from the games will be loaded into RAM/Paging files. The SSD will be able to quicker serve the RAM. But the RAM is still very much the bottleneck.
So, in short a SSD is better but than a HDD but there is a price difference. Only until recently has the price dropped on SSDs to make them even a consideration. So, basically is the minimal upgrade worth the cost for you?