I'm currently saving up for a "fun" car. Realistically I will be able to "responsibly" afford it in about a year. I grew up outside of America and have never gotten to test-drive Miatas or Mustangs, but from what I've read about them and my general impressions of them, my pick would be between those two. (Obviously I will test drive before purchasing.)

My understanding based on reading - the Miata is one of the best driving experiences ever engineered. Mustang has more power and a "masculine" feel, while being a more practical vehicle. Both cars are reliable and affordable as far as sporty cars go.

Context so people can get an impression of what I like

I currently own a CX5 and a Toyota Avalon, and I love both cars for different reasons, which is making it harder for me to decide between the Miata and Mustang. I also own a Ram truck (live in a pretty rural area now, hence why I'm able to own so many vehicles) which takes care of all of my serious hauling and towing needs. Unfortunately, I can't sell either of my cars because I let me SO drive whichever one I'm not driving.

The Reddit Mazda Glazers are right - Mazda really does have fantastic driving dynamics and nice interiors - my CX5 drives like a nimble Sedan and you can really "feel" the road. On the other hand, I do love my Avalon's powerful V6 engine, interior comfort and it has fantastic cargo space for a Sedan. I like the fact that it's quieter over bumps than the CX5 (a result of Mazda's tightly geared suspension systems).

Cars I've owned and my impressions:

Lexus CT200h hatchback - good car with super luscious interior but a pretty muted driving experience. Probably wouldn't buy again.

Honda Civic Si and VW Golf GTI - honestly these were both great and fun-to-drive cars and I never had any reliability issues with either.

Kia Optima - was ok until about 100k miles then had all sorts of problems, overall pretty bad, but it was a pretty old model.

Toyota Echo - my first car my parents got for me, small, nimble and reliable, will always hold a special place in my car but not something I'd get today.

Other factors influencing my decision:

I don't really want a soft-top convertible but the hard-top Miata's seem significantly pricier. But for that reason I'd probably steer clear of the Mustang convertibles too.

I'm open to both manuals and automatics. I really prefer manual gear shifts on motorcycles and side-by-sides, and when I drive my relative's stick-shift cars I like them, but I'm not as experienced or as skillful driving stick shift cars as they are and could see it being a problem on difficult roads and high-traffic areas on road trips. If I owned a stick shift I'm sure my skills would improve, so also open to suggestions there. For enthusiasts: Does the "fun" factor make owning a stick shift worth it?

I really prefer to buy used with cash and I hate making car payments, just a me-thing. So whichever car has a more customer-friendly used market as a plus. Based on my impressions, that seems to favor the Mustang.

I'm not the most handy guy and dislike doing lots of maintenance, so reliability is very important to me. I also don't really enjoy putting time and money into modding. Because of that reason I'd like a car that has good stock speakers and components, but I'm fine with buying used cars with mods as long as they're good.

I'm not a huge speed demon and don't care too much about top-end performance. Power and speed are nice to have, but I don't need turbochargers at the expense of reliability and simplicity and in the Mustang's case, V6 would be plenty for me, especially if it saves me money. As long as the car is generally fun to drive and has a "cool aura", and importantly, a nice-sounding engine with a somewhat quiet cabin, that's fine with me.

I value the superior practicality of the Mustang, but also I think that cargo space is pretty overrated. I drive to and from work with basically zero cargo and no other passengers most days, and have other vehicles with cargo space, so in that sense I think the Miata would be fine?

I love feeling the wind in my face, but nearly half the year is not convertible friendly and I also enjoy driving with the windows up, and listening to audiobooks, podcasts and music on long drives. For this, the Avalon is better than my CX5 with its quieter cabin and superior speaker system, and I could see that being a problem with the Miata. I also have side-by-sides and motorcycles for the full wind experience.

Although Miata and Mustang are at the top of my list, nothing is set in stone so if based on my preferences someone can suggest something better geared towards my indecisive wants and needs (Chargers/Challengers, Camaros, European convertibles etc.) I'm all ears!