Main or Alt?
Play what’s fun. The skill points will pile up as you go.
Otoh if you get a weapon that both classes can use (Bifrost, for instance), it’s account bound and you can share it.
I haven’t made a legendary, so I might be missing something, but what’s to stop you from finishing map completion and skill point accumulation with your Necro, then transferring the Gift of Mastery to your Guardian? The Bloodstone Shard (used for Gift of Mastery, bought with 200 skill pts) might be soulbound, but the Gifts are account-bound.
Unless the time spent finishing up map completion / skill point accumulation is just time you’d rather spend playing with your guardian. If you like the guardian that much more, even if it takes longer, if you’re having more fun, it might be a better choice. It’s really up to you!
Donari’s suggestion is a good one, at least if you like the Bifrost. (Focus, Spear, and Trident are also options. Just not popular ones.)
Well I want to make the legendary dagger for my Necro I think, since he is really techy looking. It would just feel weird completing the map for the first time on an alt in stead of my main.