Like people are saying, Valk goes best with Toad and Maya.
1) Most useful in PvP/Woe. You might not PvP much, but it still happens. You also BG, and Toad will help more than anything.
2) There's quite a few Demi-human monsters. Most Orcs, Four MvP's, etc, etc. It's still useful for PvM.
3) Resale value. A Toad Valk will sell extremely easily compared to a Tirfing Valk. You might think you won't resell it, but plan for the future, right?
Tirfings most often go in Stone Bucklers, and overupped Guards, due to the ease of use, and relatively cheap value.
Also - It depends on your class too. If you're a Pally, maybe you want an overupped Cross shield instead of a Valk?
Why is maya a bad choice?
30 mdef and 18% autoguard seems nice to me
Mdef isn't all that useful for PvM. I mean - it's good to have, but reducing race or property damage works better. It's by no means a bad idea - but there might be something better out there to get first and foremost.
Edit: Also - hit me up when you plan to BG. I've been meaning to try that. I'm the bloody knight sitting next to you.