I test drove both before purchasing myself a 2008 Sky Redline last month.
I actually really loved the miata. It put a smile on my face each time I test drove one. Interior was very nice on the 2008 model I drove - it had seat warmers, keyless start and a power hard top. Miata has a usable trunk no matter if the top is up or down. Handled great and felt great to drive. I owned a protege5 for 17 years and had no major issues. I feel Mazda makes great reliable cars.
The sky didn't feel as refined to drive, and the interior is basic with some odd ergonomics. Trunk space is a joke. Manual top, no seat warmers.
So in a way, the mazda is a better car, to be honest. So why did I buy the sky?
Well, miatas are so common, I know I can always get one. Once it's time to move on from the sky, I'll probably replace it with a miata (and I love the styling of the current gen!). But the Sky is an oddity. I like the idea that it's not so common. I love the looks of it too. And in 5-10 years, it won't make sense to buy a Sky, unless you are looking for a project or show car. I wanted something I could drive and enjoy and not always wrench on or baby too much. Reliability seems good, and should be until old age and wear takes it's toll. Eventually plastics get brittle or weak, parts wear out, etc. as a car ages.
But ultimately, the power of the redline is what pushed me to the sky.
While the miata put a smile on my face when I drove it, the sky, once I put my foot to the floor, got my heart pumping.
And that's what I was looking for.
The shortcomings I will either live with or remedy - I plan on putting on a tunnel brace to see if I can't get it to feel a little more planted like the miata and I want to install seat warmers as I have them in my other vehicle and love them.
After having it a month, the manual top has not really been a bother. You get used to the funky ergonomics of the interior. Trunk space has been the only thing I wish it was better at, but I just plan accordingly. I love the car and every time I take it out I feel like such a badass riding in it.
My suggestion to you though is to test drive both - multiple times. Back to back if possible.
I test drove both twice before deciding on the Sky.
However, I opted for the Redline as I wanted the power it provided.
Given you are looking at the base... well if it was me I'd probably go with the miata to be honest. If I remember right the power between the two is very similar.
Unless you really like the look of the sky that much more, the miata is a lot more refined.
As for issues, poke around the forum and use the search function. Overall what I'm seeing is not a lot of major stuff but as they age more things are popping up.
Good luck and either way I know you will enjoy your new car!
This is probably one of the most honest comparisons between the two cars.
The 2.4 (base model) Sky and Solstice were meant to be a DIRECT competitor to the Miata in all categories. Handling, performance, pricing...the 2.4 powered Sky was shooting to take on the Miata. When it comes down to it, BOTH are great choices and you really can't go wrong with either. The only category where the two cars differ is passenger comfort/convenience.
On the driver's side, the Sky is roomier. Passenger side is about equal.
Ergonomics in the Sky for cup holders and power window switches is terrible. I believe the window switches are better placed in the Miata but don't know about cup holders. They were never high on my list of variables anyway.
You don't have to get out of the Miata to operate the roof whether it's power or manual.
Trunk space is the same and fairly spacious in the Miata top up or down. In the Sky, it's bad with the top up and gets worse with the top down.
That's really it. Miata has greater aftermarket support but I don't think there is a thing you can do to a Miata that you can't do to a Sky. Ironically, putting the LE5 2.4 engine from the base Sky INTO a Miata is a popular engine swap for them. Miata's are lighter than a Sky but the 2.4 engine is more powerful than their 1.6/1.8 engines so obviously the more powerful engine in the lighter car is a bonus. Stock for stock their acceleration performance is pretty close with the edge to the Miata. Handling though the Sky has a more capable factory suspension/chassis than the Miata (the Kappa really is a scaled down Vette).
So it comes down to looks and a couple convenience issues. Both cars are reliable. Both cars are good performers. Both have an aftermarket to make them "better".
Of course if you go to the Redline Sky then the Sky has a far better performance envelope than the Miata...but the turbo cars were meant to compete with the likes of a Porsche Boxter and Honda S2000, not the Miata.
There you have it. What do you like the looks of best? Can you deal with close to no trunk space? (Think some groceries or a couple duffle bags if the top is down. A cart full of groceries or a couple more duffle bags if it's up.) The choice between the base Sky and a Miata is purely subjective.