I own a Mountune ST as a daily hack, hard to fault for a car that starts every day, is reliable, doesnt need any serious work, comfortable, heated window screen etc... and can still be fun to drive.
I'd say a E30 is a far bigger gamble money wise and unless its been well maintained wont drive anything like as well handling wise.
Fuel consumption will probably be similar at circa 30mpg, the fiesta will be a bit better but not a lot.
The cars are in the same bracket money wise, either could be worth more depending on condition. I guess yours is £3-4k ? depends on mileage etc but being written off will knock a grand off.
The sensible choice is to probably stick with what you have but then sensible is dull
