I only run OEM mags, but if you get the ETS ones, let us know how they go. Cheap training mags would be interesting. I'd still say stick to OEM for real world use.
Take a ride to the Sig Pro Shop. They are $10 less there than MSRP..... I bought a 21 rounder there for $39, then a 21 rounder at a gun shop and it was $49.....