In stock form the Miata is faster in most contests. For example, the Miata is faster by nearly a full second in the zero to 60. In pretty much all day to day driving the Miata is faster, no turbo to spool...so while the Fiat is spooling, the Miata is already gone. See for example the instrumented tests by Car and Driver of both, see the comparison in Automobile Magazine, etc. I own both a Fiat 124 and an ND Miata, have autocrossed both. In stock form, the Miata wins for autocross easily, wins the 0-60, wins canyon driving, wins around town. The only place in day to day driving where the Fiat's torque advantage really plays it's hand is in freeway driving, if already at 70mph the turbo is ready to go and passing with a tip of the pedal is a real pleasure as it squirts to 90mph with ease.
Once BEYOND stock form is another question entirely. We found the ABARTH stock tune much more lively at the Press Day driving event than what is in our Classica124. Figure Greg and Eurocompulsion gets the Fiat to beyond 200hp at the wheels, hopefully 250 eventually... and gives all versions that lively feeling we found in the ABARTH 124 with Sport button engaged. At that point it will be an interesting contest, the Fiat will still be heavier but should have more torque everywhere. Note we took 52 pounds out of the Fiat in one day (light battery kit and front brakes), so the extra 100+ pounds of the Fiat can be cut in half easily (though same mods can save 39 pounds on the Miata which uses lighter stock battery). The Miata also responds VERY well to mods, with stock 2.0 at an exotic 13 to 1 compression it just needs our longtube header and tune to reach over 190 hp at the wheels, and over 190 ft/lbs torque (see dyno below of our ND with longtube header and tune compared to our ND when stock). Thus our ND is up from stock by over 50 HP, over 50 Ft/bs of torque...and still using stock intake. Will test our Fiat124 in stock form on the same dyno and overlay the stock graphs for comparison and do it again once the Eurocompulsion solutions are available so we can compare modded ND to modded Fiat124.