Crippler for sure...!!
Both DPS or Burst!
Experiment on this if you like and then prove it:
1. Go to a dungeon with the exactly same team for 2 runs.
2. First run: full invest at double blow
3. Note your results at the end of last boss. Print screen it, so you don't have to use papers, excel sheets or whatsoever.
4. Out from dungeon: Reset skills, go for full invest for Crippler. Don't forget to ask your team to wait for you to reset and setting skills on shortcuts.
5. Second run: Then do point 3 again.
6. You will got results: Crippler is best overall burst damage and DPS.
I did this on my SW: ValkyrieClaw (RG).
My recomendation: 5 points on Crippler; 0 for Double Blow.
Crippler works best for PvP too, not only PvE.
To get the best steady team for experiment, you can ask regular active players on your guild.
Or if you are OP player, you just need to "borrow" a party. (asking someone to party you, and then you go to the dungeon alone until your experiment completed).
On PvE negative notes, Crippler has long duration cool down (9 secs). You will "feel" clumsy when all of your skills are on cooling down state. But don't worry, it is not your style which determine you are good or not, but your rank on DPS board will do.
To use a balance of Double Blow and Crippler is not recommended. When you did the experiment, you will find: Full Crippler wins the highest DPS amount compared to Double Blow, or a set of balance between those two.
To get factual result; doing an experiment is a must, not only calculator based logical conclusion. There are a lot of factors out there which influence the outcome produced. (opponent DEF, VOID, element resist, your balancing actions to refill HP/SP, cooling down state, buffs, de-buffs, etc.)
There are a lot of facts you will get which give contrary results against calculator based logical mind set.
After you got your results; I would like to recommend you to use: seer/envoy for earrings, scout/killer for necklace and trinket, and then seer/envoy for rings. That will make more difference on your crippler (and all other skills).