there are quite alot of factors to consider in this comparison..
Maintenance is always cheaper for petrol engines then diesel but on the other hand diesel gives better mileage then petrol. There are limited 4.0 Prados in market and they all have a price tag of 5 million(Tz, dual Ac, height control etc) but on the other hand a good LC 2006 model costs 5.5 to 6 million but there are many ranging from 5million onwards.
biggest problem in buying these vehicles is the condition as there are many abused over used vehicles in market but very few which are worth buying and since every single of these vehicles are driven 150k-200k kms but meter is reversed to show they are 70k. No matter what you do you cannot make an abused vehicle feel like nicely kept vehicle..
So who ever is in market to buy some 4x4 and have 6 million to spend i would advice him to simply buy the 2010 150 series prado 2.7 2trfe and enjoy it.. and second option would be the Land cruiser if its in Good condition