Out of Thousand Swords and Raise Your Sword, you will like Raise Your Sword more, most likely.
RYS is more "viking" and bombastic and in the vein of Creed of Iron and Memory and Destiny. The first song on RYS is friggin' fantastic.
Thousand Swords is commonly referred as the best Graveland. I don't see it. Good, yes. Their best, no. Following the Voice of Blood is easily my favorite. Like Doom said, it's not easy listening. I admit I didn't fully appreciate Graveland until after a good 3-4 months after I bought the albums.
The Fire of Awakening is also a good choice if investigating them further.
If you dig Immortal Pride and Raise Your Sword, it's a given you'll enjoy Creed of Iron, M&D, TFoA, and probably FtVoB. The Blood of Heroes EP is OK, not really essential to the casual fan. Raiders of Revenge (split with Honor) has some really good Graveland songs, but the Honor songs are just ho-hum nazi oi metal or something with awful vocals.
Epilogue/In the Glare of Burning Churches, Carpathian Wolves, The Celtic Winter are all atmospheric black metal with zero of the Pagan spirit from the later albums. If you've heard Veles, then you get an idea what these sound like, though they are much better than Veles.