Well, the really easiest way is to call it an artifact, give it stats without assigning any costs and treat it as a priceless thing.
If you really need a cost though, both solutions pose some problems, so I'd choose the system that just better suits the rest of the game.
- Advangages don't mesh armor pen. and ST-based damage well. If you want to have a set value indifferent to user's ST, use Modifying ST-Based Damage, p. P146 and calculate it for ST 27, which is the maximum usable ST for naginatas (see ST (Strength) on p. B270. You can only add lifesteal atop of ST-based damage, really
- Spells don't have a set price for granting an extra life (not that I know of). That's something you'd have to evaluate yourself, comparing it to Blessing and Wish, for example