SSD is about three times the speed of a conventional 3.5" HD (2.5" HDs are a little slower than 3.5" conventional HDs).
Whether you need the performance difference is completely dependent on the scope of your projects.
If you're not working at higher sample-rates... and not working with large track counts, then the 2.5" conventional HD will be fine.
If you need higher performance, definitely go with the SSD.