I will add 2cents if it's not too late.
Before I bought my Q I checked out all cars you mentioned but GS400.
G300 is like a Camry, which is not bad of course. Cons: it's not spacious, in fact it looked less roomy than q45 (it is narrower), poor rear view (high trunk), mushy suspension and it is very _slow_. Especially comparing to Q.
LS400 is a truly luxury ride with all that belongs to it: dead steering, soft suspension. Pretty spacious in the back but not that much over the Q. It's not that fast either. Nakamichi did not blow my away, btw.
3.2RL I like the way it looks, really. Very classic. Good for a woman I think. But it's FWD. I did not want it. It felt faster than gs300.
i30 is what Maxima should be. And it really is, cause that way Maximas are sold everywhere but in US. Comparing to all other in the list may be less comfortable ride, but better control, road feel and good pick up. You can't make fair comparison with others cause this car one rank lower than Q in all aspects. FWD.
3.2TL is Accord, which is not bad again. FWD. Fair to compare with i30 not with Q, LS and GS. I woud prefer I30 at least for the better engine.
So, I ended up with q45 touring and very happy. No problems, just maintenance up to standards of this board. It is pefrect interstate cruiser. We made over 1200 miles drive with my family through Texas and did not notice it. It was said true about comfort on rear seat: my wife is all the way with baby back there and always sais it's very comfortable. Keeps beggin me to take her on trip to Florida in this car
I think overspent my 2 cents

Hope this helps.