Is LW worth on MF? Or just stack another BT?

............ DO not buy 2 BTs... such a waste of gold. There are several other items that give you 80 ad if that's what you want. Armor pen is always a good thing however.

Don't stack another BT, half your gold will go to waste since BT's lifesteal is a unique passive now (meaning building 2 BTs will get you +160 AD and +20% lifesteal, not 40% lifesteal). Last whisper makes the first BT worth more effective gold by increasing its damage, while a second BT is worth half as much now since the changes to it.

The most popular item across the board for ADC's will be

IE -> SS

but for MF you can start BT --> SS --> LW

or BT --> SS -> Ghostblade if the enemy team is all squishy solo queue noobs / assassins

LW is fine one her. Technically ALL ADC's should built it eventually unless their name is Kog. There's some math somewhere I believe that because of how much mixed damage he does, getting a PD instead of LW allows him to do more DPS overall. Outside of that most ADC's should always build LW unless for some reason the enemy team builds no armor. Technically your build should/would be alone the lines of IE > SS/PD > LW, but if you're REALLY far ahead and/or just pushing towers, getting BT can work better. Also there's no point in stacking BT anymore so don't bother.

I prefer going Full AD...and if the teamcomp can use it add a Black-Cleaver for Arp, as she can easily stack it on all opponents with her ultimate.

Black Cleaver Last Whisper master fortune race.