If you plan on spending a lot of time on party-based PvE, these two skills might be worth it. And that may be the case once you get past level 30. We've yet to see what that looks like, but my guess is that you're going to be spending a lot of time in groups, since there's a lot of open world content.
Shadow Dance is pretty good. The problem with it is that you have to get Crippler up to level 5 to use it. Crippler is ok, but with our limited points, it's hard to justify 5 points there.
Also, the items in the next tier tend to do a little more damage. I don't have the info up in front of me, but I think Tier 4 has slightly longer cooldowns though.
Anyway, it's all about points. You probably don't have enough points to get both Tier 3 and Tier 4 up to a useful level. Tier 3 is better for party-based PvE. They're both decent for soloing in PvE, but Tier 3 might be a tiny bit better for that. Tier 4 is better for PvP.
Since most people care more about PvP, and when they aren't PvPing, they tend to be soloing, Tier 4 is generally preferred.
Server: Andromeda - Faction: Royal Guard - Guild: Guardian - Name: Balina