I’ve talked many times about SSD drives. They are incredibly fast (many times faster than a mechanical drive), but cost more than a traditional mechanical hard drive. So the ideal setup is to have your operating system installed on the SSD drive and your software/data on a mechanical drive. This gives you the most bang for your bucks.
If you are wanting an SSD drive today, the Samsung 840 Pro 6 GB/S SATA drives are the best right now. They come in three sizes: 128 GB (approx $128), 256 GB (approx $225) and 512 GB (approx $468). Note that the prices listed are the prices at Amazon as I write this. They could go up or down at any time and it is completely outside of my control. In my opinion, 128 GB is really cutting corners for today’s operating systems and you would likely run out of space before too long. For only the operating system, 256 GB is just around right. If you want to partition the drive to install both operating system and software, then you’d want the 512 GB drive.