Ssd's are great. Totally silent, extremely fast, good ones last for years, longer than mechanical drives. But they are small. Unless you can afford a huge 1-2Tb, for most they are cost prohibitive. Windows, with all its attendant crud like Office, Adobe, Java, Antivirus, Photoshop etc that really benefits from native ssd speeds, can easily soak up 80Gb of a 120Gb drive, not leaving room for much more. A 250Gb ssd has more room, but is 2x the price of a 1Tb Hdd.
Mechanical drives are noisier, but offer the 1 thing a ssd doesn't have. Space on a budget. At $50 for 1Tb, you can load a massive amount of games, photos, media and still be pressed to fill it. Even if you do, another $50 drive is child's play to add.
So the real question becomes how much space do you need, not just want. A pc just for web browsing doesn't need much, a pc for movie storage is considerably different.