well you can't say regardless of miles because that is a factor when purchasing a car , the Grand Marquis was probably used by someone older ((more responsible to keep up regular maintenance)) plus the cost of repair is cheaper )) it has probably been driven responsibly
The lumina more or less has probably been driven by younger drivers ..so it has been driven a lot harder which means it is probably almost worn out ., and no regular maintenance , the lumina is front wheel drive ((more moving parts)) plus driven by a younger person = more problems plus lumina's are more costly for repairs which will be more frequent due to younger drivers ..
The grand marquis is an 8 cyl engine vs. lumina 6cyl ( maybe )
the difference in gas usage is nothing compared to the repair bills on the chevy..
plus I have rarely seen a lumina go over200,000 miles they normally start smoking anywhere from 80,000 to 150,000
So based on that I would say go with the grand marquis if taken care of it will last longer than the chevy , has a lot better ride
the marquis is the exact same car as the crown victoria ((just different front & rear ends ,and instument pannel )) it hasd the same drive train , so the marquis is a more top of the line vehicle , and the lumina is more bottom of the line