Have you looked at how partner flights credit to JL or NH accounts? You may find that one has better mileage credit on certain partners (maybe those that you'd fly more often than other) and that could help your decision as well. Say that JL Partner A only gives 50% credit towards your JL account on the fares that you typically book but NH Partner B gives 100% credit towards your NH account on the fares that you typically book. In that case you'd probably lean towards NH. Don't look just at earning on the primary carrier but also at earning on the likely other carriers that you'd use.