since this is the GM forum and this thread is more about aluminium vs iron

supposedly the LS1 and the LQ4 (LQ9) cost the same
which one should go in the miata ?

good old weight vs HP question .... 100hp vs 100lbs

if they cost the same (good luck with that!) then LS1 is clearly the better option. However the iron blocks are cheaper and if you can run a t5 then it's way cheaper. That's why I'm using a LR4 paired with a t5. that said wiring is a whole different animal. I don't have any good info on that though, because carb.

The overall weight difference is less than you think. Do a search and you will find that a member as actually weighed the blocks.
At this point you have two engines that will both work very well in your car. If price is not the issue and you can live with the weight difference then the deciding factor has to be the horsepower output you want. At higher levels the aluminum blocks run into water jacket issues. The cylinder liners can walk a bit causing leaks and you know what anti freeze does to bearings....boom. There are a lot of guys running high HP aluminum blocks but it is easier to safely get the same level or more out of the cast iron block.
For what it's worth, I was running a 99 Corvette LS-1, good headers, a large free flowing intake, and a good tune. The engine was spec'd to be putting out about 400. With a T-56 ad 3.73's the car was 0-60 in 4.80, 13.4@109 1/4... the throttle was only opening 2/3 due to not having the pedal adjusted at the stopper. I had an absolute blast with the car and it was everything I expected and more. For a good solid build there is no need to go crazy with spending money for HP. The chassis can take it but a little over stock is enough.
That said, I am going balls out on the Exo just because I can......excess is a nice place to start!