My experience was the same. A while ago I ran out of primers for my 9mm (this was during the period where primers were almost NON-EXISTANT) and a local shop got some in, but they limited buyers to 1,000 of each 'type' of primer. I bought 1k of SP and 1k of MagSP (the MagSP were almost $10 cheaper for some reason) hoping that the MagSP's would work out okay with my handload. Turns out, the difference between the two was barely noticeable. If anything, the MagSP's IMPROVED the accuracy of my load (by a small amount) and the only thing I noticed was a small difference in the way they sounded compared to the regular SP's. I had no pressure signs/flattened primers, no light strikes/failure to ignite, etc... In the future, I will (happily) purchase whichever variant is available, and if the MagSP's are cheaper guess which ones I will choose?
BTW: The SP's were CCI500's and the MagSP's were CCI550's.
-Kyle
Thanks for the info, I'll try the SP mag primers if I find one.