How old are you / whats your insurance rating like?
3.9 is the 'nicer' engine, 20bhp more on the quoted output, but an awful lot more low down 'stomp', though entirely dependent on condition / set up.
3.5 is easier to insure, particularly for younger drivers; 3.9 due to capacity and class of car when new falls into a much higher insurance group that some companies wont offer to drivers under 30, apparemtly.
3.5 usually has flapper EFi if its an injected engine, or carbs, where 3.9's tend to be hot wire.
No significant advantages one way or the other; carbs motors are a tad thirstier and a tad less powerful than EFi's, of the EFi's, hot wire is a bit eaiser to get bits for and has a couple less common problems......
Again, end of the day, condition is more important than spec.
You say both have LPG, what systems do they have, where are there tanks?
Boot tanks rob space and often stop you folding the seats down to avail yourself of the full benefit of the 'estate' car, under sill tanks rob ground clerence..... once better off orad, onces more practical for doing the 'chores'......
Open loop LPG is not that refined or responsive, but easy to set up and generally reliable, closed loop more complicated but a bit more responsive and a tad more ecconomical if you can get it set up right.
Which one is in better nick vis rust on the chassis, the sills and in the crooks and nannies of the body?
Have you checked all the usual suspects, like behind the sill covers, in the rear wheel arches and boot floors, and around the inner wings and body mounts at the out-riggers?
Which is tidier / has more of its toys working?
and if all else fails; and insurance not an issue; go for the 3.9 for the simple reason that they made more of them with fewer variations so less niggles with obsolete or difficult to find bits; engines got more go, and its probably the younger car!
