Miata for sure they're smaller and would be better on your fuel economy and they still make all kinds of parts for them since millions were sold. Only bad thing is its a 99 and pretty old so it's likely both cars will need some kind of work. Might want to try and find something a bit newer if you can. Either way before you buy have a mechanic you trust check the car over first.  

i like mustangs and would go with that one. both are a bit old so it boils down to what you prefer.

I'd rather have the extra room. bigger inside, bigger trunk,more leg room.

just my 3 cents.

It's really impossible to say without a great deal more information. Either one or both could easily be a ticking time bomb of problems since they are used, 20+ year old sports cars. You need an honest mechanic to check them out.