The lug will probably have the larger thermal mass than the wire, so having the tip of the iron in contact with the lug would be the best option.
Whether the iron is on the same side as the solder or the opposite side, I would put the iron and solder on the same side, then when the solder melts it makes a good thermal contact between the iron and the lug and the wire.
The important things are:
1 all the components are up to the correct temperature
2 the solder makes good contact and "wets" all the components
3 there is the correct amount of solder on the joint (not too much, not too little).
JimB