Well the advice I was given 5 years ago when I bought my 4.6 was that the 4.6 was a (much) better performer, but that it tended to run "hotter" and was thus potentially likely to have problems with LPG conversions, due to the low tolerances - in other words, if you want to run a P38 on LPG, I was advised not to do it on a 4.6 as it would just be a metter of time before major engine problems arose UNLESS you do a complete engine refit like tophat liners or a Coscast block, at the time of fitting the LPG system.
So loads of people on this forum may shout "bollocks" at the advice I was given, but then I've had the Rangie over 5 years now and over 120 000 miles and the engine has stayed sweet, so perhaps the proof of the pudding is in the eating as the saying goes.
At nearly 150 000 miles I suppose she could pop her clogs any time and is using a bit of coolant now, but the rad is a bit suspect, but I am eternally grateful I resisted the temptation to fit LPG - the "saving" from the LPG would have cost me a fortune in a new engine etc and the 4.6 Special Edition I have has a very high interior spec which I believe the 4.0S don't have.
Horses for courses, but good luck with whatever choice you make.