My preferences:
1) Physical bitcoins, because they look very nice
2) Litecoins, because they will grow
3) Bitcoins, because that is where it all started.

BUT I wouldn't consider 2, and 3 as an investment. seems too risky to me at the moment.
While I have some it will not be fatal for me if both of them drop considerably.

BUT I'll keep casascius (when I get one) for sentimental reasons