SSD or upgrade RAM (slow hard drive)
I had installed on my MBP mid 2010 a Samsung SSS 830 250GB. 
With SSD you will solve all the problems caused by HD latency and speed (in my case speed up is around 10x). 

Of course you also take advantages on speed when you use swap memory with an SSD. It's like when you write and read something from a disk.

In summary, if you want a huge speed up go for an SSD.
I think the best SSD) now is the Samsung 850 Pro 256GB  (storage / price. 
RAM upgrade is required only if you do intensive computation / gaming / image or video editing.