*Your question can't be answered properly because you do not specify enough information:
1) What are your STORAGE requirements? (i.e. is your 500GB drive almost full?)
2) Is the upgrade primarily just for a SPEED BOOST?
I usually recommend people who want a good combination of large storage and speed to do this:
a) 120GB SSD (i.e. OCZ Vertex 4 or Samsung 830/840) for WINDOWS and Apps.
b) 1TB or 2TB hard drive for Downloads, Media, large STEAM games folder and BACKUPS of your C-drive
PRICING:
Hard Drives -> slowly dropping, have mostly recovered from the flooding price hike
SSD's-> rapidly dropping, have dropped to about a THIRD of the price AND have gotten better/faster at the same time in a TWO YEAR PERIOD.
Current SSD's cost roughly $1 per Gigabyte.