As a person who actually done both swaps, I can give you some insight into both racks.
I started off with the miata about 4 years ago because while I was at a junk yard finding parts for my Z, I saw a maita and the rack LOOKED very similar to the stock z rack! but it was 3 inches too long! I thought I'll just offset it to the passenger side to compensate for the extra length.
So I bought it and proceeded to do the modifications to my car to install it. Now mind you the passenger side tierod assembly was 3 inches shorter than the driver side, but In my experiences driving it around town, I found no adverse affects of the different length tierods. The feel around city street speeds was great, but on the freeway it was a little twitchy. But I got used to it.
Then I got into an accident alost 2 years ago and almost totaled the Z! While it was in the body shop I started seeing pictures here on HybridZ of the subi rack and how it was a much closer match fit to the stock Z rack. So I purchased a 2003 forester rack and proceeded to do the modifications to the crossmember to install that rack back into my Z while the body work was being done to my car. I was lucky that the body shop owner allowed me to work on my car while my car was in his shop! You know liability issues and all! Anyway, my car was finally finished and now I have to admit that the subi rack feels better than the miata. Although it was a harder install that the miata rack. The mods were more extensive but I think it was worth it. The steering around town has a better weighted feel to it and the freeway twitchiness is not as bad. I think you'll like the feel of it! I hoped that answers your questions! If there are any particular questions that I can answer , just P.M. me.
Happy motoring!