AFM uses a flap system where it measure the volume of air.
MAf uses hot wire/hot mesh to calculate the MASS of air and works much fatser than an AFm and measures more accurately. Its also non restricitive and is basicaly near anough a hollow tube.
Gains seen before on a 2.5 are a minimum of 10bhp and 10 lb.ft torque peak and more throughout the rev range.
Well worth it especially combined with a 6 branch and BBTB.
I'd look at the cost of the MAF compared to a mappable ECU setup as there might not be much in it, in which case you may well stand to gain more from the latter