As you already should know SSD much much faster than traditional hard drives. And if you would like to buy one which should you buy?

Firstly thing, you have to figure out how much space you need. Usually these days I would recommend having around 500Gb SSD drive. You will install Windows and most software that you use everyday on SSD and rarely used stuff or stuff that is ok to be slow or just huge stuff on hard drive. If money is big issue, then you can buy 250GB and depending on how often you start computer per day you can install Windows on SSD or hard drive. I would not recommend buy 128GB drive unless money is huge