If you plan to do some serious off-roading then it's a no brainer, go for the 70. However, you have to remember that the 70 is basically a tractor with A/C (you've owned one so you know what I'm talking about), the 90 is a little more comfortable and convenient.
If I were you, I'd go for the 70 and start adding accesories to make it more "civilized" and if the suspension is too stiff for you then get an OME kit that makes it a lot better.
I actually was in the same boat as you, the Prado, called Meru here, only comes with a 4cyl 2.7l engine and that along with the IFS was the deal breaker for me, once you've driven a Machito 4500 EFI you just can't go back to anything with less power. I'm currently waiting for my Machito, they are hard to come by because of the high demand.
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