ThanksSSD is several orders of magnitude faster. Both my PCs have them and I would not have a system without one.
The easiest thing is to buy a sata SSD that is big enough for all your storage. However, that is more expensive than getting both. If you store lots of pictures/videos then the recommendation is to buy a 250GB SSD and an appropriately sized HDD. The operating system and programs go on the SSD, all data and games that don't have long load times go on the HDD. You repoint all your my documents/my photos/iTunes etc to the HDD: