My noticable difference?
Potatoes are just a meal/snack thing, sweet potatoes can be meal, snacks or desserts... they have a "sweet factor". You can bake them like chips, cookies, stuffed with dates, honey and chocolate, etc... Top a sweet potato with coconut butter? HNNNNNNG.
As for potatoes? Well... potato. That's it, really. You can make chips, add sour cream. But only really savoury foods - which don't get me wrong, I love potatoes.
IMHO just depends on personal preference of taste and if you can afford it or not. "Do I enjoy sweet potatoes, can I fit them into my budget, and will they be worth the money?" is the only real question you should be asking. But I'm sure (knowing this forum) you'll get some guys/gals coming in here to post their sciency links. :P