Now, here's the scary part, there are a handful of malware that is specifically targeting POS systems. There are the Dexter, Project Hook, Alina, ChewBacca, JackPoS and VSkimmer POS malware to name some of the more prominent ones. Then there are the credit card skimmers so if you're using your card at a gas station, be careful.
An encrypted MSR (magnetic stripe reader aka credit card swiper) can defeat most of these malware but most MSR's are unencrypted. You can literally open up notepad on one of these, swipe a card and see the full card data which includes cardholder name, card number, and exp date. Won't help you from a skimmer though.
As for debit vs credit cards, credit cards are much safer. There's more protection and less hassle if fraud does occur.