Do you already have the models?
If you do, take them both out in battle and try them out for size.
Then I'd go with preference. Everyone will normally say LS. Well I like both. I find speeders pretty weak when it comes down to the nitty gritty. With the Ass cannon all your doing is rolling dice and hoping for a 6. So you throw them intoi somewhere they shouldn't with the hope the get rending. With the attack bike you have to think more clearly. It'll never have rending, so you start to think tactically. Where will it be most effective and how can I cause most pain... and fast.
So depends on your tactics. If your running bikes already, go AB... If not consider LST
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