DT88 - If you were to order one, my guess is the dealer is not going to cut you such a great deal. I could be wrong. I don't even know if they have the ability to "order" one. Or if they would just "find" you one. You ask a lot of good questions. I don't know which car is "better." I'm a big Toyota fan, having worked as a design engineer for Toyota (and loved it). But I really don't know much about the iA. I know a fair amount about the Mirage.
Here is the Mirage as I see it: It is purpose built. It's purpose is: TO BE INEXPENSIVE. Various materials selected are just plain cheeeeep. Does that bother me? Heck no. Certain equipment on the car is about as inexpensive as it gets. The wheels and tires that come stock ... cheeeeep. Yea sure, if you buy the Dunlops as replacements they're not that cheap. But I bet Mitsubishi gets them for a price so low we wouldn't believe it. Does any of that bother me? No. I love the fact that the less I spend on the car, the upkeep, the care and feeding (gas), etc. the less I am paying to our big fat wasteful gobment (in taxes). Less gas purchased, less gas tax wasted. Etc. I would have bought a $2,500 Tata if that had ever materialized. Or a $7,500 Elio if that had materialized.
You mentioned the clunky shifting. Well yeah, that's a Mirage thing. Especially 1st to 2nd gear changing. The clutch, I don't really think there's anything wrong with the fact that when I go from driving my 3/4 ton diesel powered Ram with a 1,000 lb-ft clutch, to stomping the Mirage clutch pedal through the floor. I mean, the engine makes like 7 lb-ft of torque. Does that bother me? No, I think 7 lb-ft is excessive. Give me 6 lb-ft and 5 more mpg I say. That's what the car is for.
Whatever mpg you get with the iA, if you drive a 5MT Mirage under the same conditions, it will get better mpg. Do I wish it had one more gear ... oh that's a tough one. With the current 5MT, doing let's say 75 mph, the engine is capable of putting out ~95% of it's max torque at that rpm. That is a good thing in a couple ways. 1 - It pulls hills with ease, I've never had to shift down to pull a hill. 2 - I.C.E.s actually burn fuel very efficiently at or near their max torque rpm. That's a complex thing though, it could be that it is only efficient at or near max torque with a wide open throttle. In any event, my issue with giving it a 6 speed, I wonder if dropping too much rpm might create instances of needing to downshift to pull a hill. That would be very annoying. My wife had a 1990 Celica years ago. On the interstate it would turn like 4,000 rpm at 75 mph (5 speed). It was buzzy and annoying. But the 1.2L 3 cylinder, it has a very pleasant hum at 3,500 - 4,000 rpm. It is very pleasant indeed at that rpm. So for me, I'm fine with the 5 speed.
You have to be honest with yourself, which one are you going to be more satisfied with long term? Noone can answer that but you.
View my fuel log 2020 Mirage ES 1.2 manual: 42.3 mpg (US) ... 18.0 km/L ... 5.6 L/100 km ... 50.8 mpg (Imp)