Claws would be the way to go.....
Claws pros - fast activating attacks, with low endurance costs and reasonably quick recharges. Ranged attacks in Focus and (to a lesser extent) Shockwave. Claws cons - lack of a big hitter attack.
Ninja blade - slower activation, but has two biggish hit attacks, plus divine avalanche to increase your survivability at the expense of damage output.
Both have cool Assassins Strike animations, but i prefer claws - it just looks plain evil.
I'd go ninja blade every time.
Claws has quicker recharge and lower end but lacks the damage of Ninja Blade. Given that a stalker is all about hitting high damage attacks then ducking back out of combat to drop back into hide, the damage is a lot more important.
Claws would be better for prolonged battles to give you lower end a more constant attack chain but stalkers aren't really designed to go toe to toe for prolonged periods
Personally ninja blade for me.
Ninja blade is a slow starter but you have 2 excellent final attacks and I can scrap well in most fights if I want thanks to divine avalanche, which fills in between the big attacks.
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I'd go ninja blade every time.
Given that a stalker is all about hitting high damage attacks then ducking back out of combat to drop back into hide
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Which is one way of looking at the stalker role - I personally believe that stalkers are perfectly capable of scrapping very effectively, and claws is very good at scrapping.
PVE wise - BU + AS the boss (maybe a couple more attacks depending on rep). Placate a lt, AS. Scrap the other LT, or two minions, while placate recharges. The other alternative is to bail out to get back into hide (say 10 seconds as a reasonable timeframe). In those 10 seconds, I can drop a couple of foes.
PVP wise, as I'm not EM, Assassins Claw is a big part of the game, and any followup attack to get around the no-oneshot code. If facing multiple opponents, or a tough opponent, having good sustainable damage is far more important, with placate/AS providing burst damage. Unlike ninja, I've also got range as an option to catch runners.
I went /dark to allow me to scrap a little more, but conventional wisdom is that /dark is rubbish for stalkers too - which it is if you try to play hit and run. It's very good if you scrap though (and tremendous fun).
YMMV of course, but i just think nin/nin is awfully dull (especially when you consider that ninjas are entirely lame in comparison to say, pirates 
)
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Claws cons - lack of a big hitter attack.
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AS anyone?
Im fairly certain that eviscerate compares ok to other 'bighitters' apart from energy transfer.
Claws scrapping is all about focus and eviscerate. Focus makes a great followup to the AS with its range and very fast activation.
Personally Im not saying that Claws is great - I made a claws stalker because
- it is good 'enough'
- it is by far the coolest stalker primary
- to show it can be done 
If you want to err on the side of FOTMness, take ninja blade.
I'm with Hammerfall and the Cap'n on this one. I have no experience with ninja blade but I can say that my claw/sr (lvl 23) is alot of fun. If you want burst damage, rely on AS. Focus is a good second. Hidden, Buildup + Focus = One shot on a +1 minion. Eviscerate has slightly higher damage then focus but doesn't garantuee a crit from hide since it's a cone. Looks very stylish btw.
I also rebuild my Claw/Sr in the test server RV event. Only hide to ...well hide me and I did fine. Got plenty of kills in and had lots of scraps with scrappers (of which I won half). It appeared having a good ranged attack is what really makes a difference in the 'to be seen or not to be seen' discussion. As long as I could attack first (from a distance) I'd be fine .
I've decided to go for Claws/Nin, mainly because I already have a Scrapper with Katana, so a bit of variety would be nice. Thanks for the help guys .
I'm planning on rolling a Stalker, something of a ninja... so he will be */nin. Now at first it seemed obvious that nin/nin should be the direction to go, however I also thought that Claws may be a good alternative and possibly a little different from the average ninja. So in the end it's come down to which one is the more efficient set, Claws? Or Ninja Blade?