A Lightning II weapon is more damaging than +1 transmuting. It's also more damaging than +5 transmuting. Or even +5 transmuting of pure good.
The transmuting gives you +15 damage. Lightning II has 2d6 Holy + 1d6 Shock + 10 Lightning Strike + some more Shocking Burst/Blast. That comes to 3d6+10 ~=19.5 extra damage, so that it is ahead of +5 transmuting by 4.5 damage even before you factor in the burst, blast, and larger physical damage die.