pretty much this. No reason at all to really take any of the other missles. unless you just want to have fun with it, then go crazy and get implosion missles!!! But those are not competitive. The missles come for free in the fighter vs having to pay for them on the bomber, also the bomber does not have all the missle options.
Converting a fighter model to a bomber? They are most likely coming out with a model and it will be sick so no reason unless you can't stand not playing with it right now.
I run one fighter with Night shields, filckerfield and upgraded cannon. Best outfit I think for it. Only personal choice really is Dark lances or disentigrators. I like the flexibility of the lances. While the cannon does nothing against armor, the lances do nicely. And against infantry I like having the lances for causing instant death to say......nobs! lol. There is valid reason to have the dissies for 6, str6 Ap2 shots against the plethora of grey knights though!!!!