I’ve just had another occasion to compare directly two pretty similar machines: A recent off-the-shelf MacBook Pro (I5, 4 GB RAM, 500 GB SATA) with my normal working MacBook Pro (I7, 8 GB RAM, 500 GB SSD). Falling back to the slower machine, and not using all the RAM and no processing intensive applications, I can only suggest that should you want to really speed up your machine, go for an SSD. It is not just about starting programs faster. The whole machine behaves completely differently. It just feels as if there were years of difference in the performance of both machines.
Bottom line: ceteris paribus, go for an SSD.