This depends entirely on how often your warlord plans on attacking, charging, and to some degree, how your party is built.
If your warlord is lazy or mostly lazy - as in, the sort who grants attacks more than half the time - neither of these is a particularly good weapon choice. You might look at something like a chieftain's weapon, or something else that boosts your granted attacks.
If you have other cold-based characters in your party and your warlord will be attacking regularly, then the perma-frost combo (frost weapon + lasting frost + wintertouched) is a good choice. Frost weapon and lasting frost are both classed by The Warlord's Handbook as light blue in general feat rankings for warlords. Wintertouched is black, but that's partly due to it's uselessness prior to late heroic or paragon. (In character optimization threads, light blue is the second highest rating given to items, whilst black is mid-range).
If you don't have other cold-based party members, or if you are otherwise fairly optimized for charging, then the vanguard weapon is a good choice (are you going to charge several times/encounter?).
Other things to consider would be if one of your party members is going to take the Morninglord paragon path, then a weapon that deals radiant damage should be on the menu (Sunblade, Radiant Weapon).
There is not a right answer here, and as you are a leader it largely depends on your party composition and how you perform your primary role.