Both cover similar ground, but from slightly different angles.
Mythic Iceland covers Iceland in more detail and has a lot of information on the Western Lands in North America, but covers Scandinavia and the British isles in about the same level of detail as Vikings of Legend.
Character generation is similar, as Vikings are Vikings, but Mythic Iceland has a bit of flavour that Vikings of Legend doesn't.
As for magic, Vikings of Legend covers Runes and other magic in a comprehensive way, but does not give powers to deities. Mythic Iceland also covers Runes in a comprehensive way, but gives powers to deities,
Both detail the magical races of elves and dwarves, both have stats for mundane and magical creatures, both have stats for Viking ships, bith have descriptions of the culture and of how Viking law is implemented, both cover the Eddas and Myths of the Vikings.
Mythic Iceland has some detailed locations, with scenario seeds and myths. Vikings of Legend has a number of scenario seeds. Mythic Iceland has some full scenarios.
Mythic Iceland also has a section on Cthulhu, which isn't to my taste, but might be good for people who want to use the Cthulhu Mythos with BRP.
So, which is better? They are about the same, really.
I marginally prefer the Legend system over BRP, but use a lot from both in my games. I just about prefer Vikings of Legend, but it is a very close call. If you don't mind using PDFs, I would suggest buying both as PDFs and using them both in your setting.