ALL Clan / Country affiliations aside...

What colors do you wear most often? What colors do you look best in?

If you look good in / wear a lot of Red (especially shirts) or black shirts with Red Writing, I'd say go for the Wallace. Also, it's a "stronger" tartan, so you have to enjoy wearing color.

If you look good in / wear a lot of Green / white / gold, get an Irish National.

If you look good in / wear Navy or dark green or just like darker colors, go with Blackwatch.

Bottom line... YOU are the one wearing it! If you have a favorite color or have a LOT of t-shirts with a certain color in them, try to pick a tartan that will match the rest of your wardrobe the best.