Depends what's in the rest of the list, but I think Grav's a better fit. Grav is a very strong weapon that you can't get an awful lot of very easily since it's the super awkward Salvo, so putting it on Bikes is a good shout. Never really been a fan of Bikes myself, but when I run them I put Grav on 'em every time.
if you aim for a white scars army with 5 units of bikers and scarcely something else i'd say that you can safely field all grav PLUS some assault bikes with multimelta. Also, land speeders with multimelta and meltabomb scouts to clear heavier targets can be useful.
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