Yes! Both are right but in situations..If you did a favor for me for example, I'll thank you and say "Shokran, Honest" but if I was writing a novel in formal Arabic I'll write "Shokran Lak, Honest شكرا لك"... you get it? what we use normally is just "Shokran شكرا" in our daily talks.
I hope I didn't get you confused now! If you don't mind me asking though, there are loads of Arabic words..why that specifically?