SSD is faster than HDD and in general you'll see faster boot and faster loading times in games. Yes you can use SSD instead of HDD but they're quite a pretty penny right now so it may be heavy on the pocket. 120GB is $80ish. Right now, if budget is tight, its best to house the OS on SSD and some major games or programs and use a HDD for other data.
SSDs are much more reliable than HDD and more energy efficient too.