4nine is at smallest 49 millimeters while the LPG is 56 millimeters, but with 4nine, you are able to use all 4 battery types, 18350, 18490, 18500 and 18650 while as LPG v2 uses only 18350, which can be an issue with rebuildables + the atty shroud might be an issue with attys with lower airholes restricting the air allowed to cool down the coils.
Personally I would get a 4nine in either copper, brass or stainless steel, because it can use a wider variety of batteries, but the button isn't the best one in a 4nine.
TL;DR: If you want to use a wider variety of coil resistances and possibly sub-ohm, get the 4nine.
If you are interested in a small but not as powerful setup, get the LPG.
I'm trying to say here that you can not subohm with a 18350 battery, but you can with a 18490, 18500 or a 18650 battery, as they have higher amp limits.