This might be too late for a reply, but to answer the general question of SFX vs ATX:
To paint broad strokes, SFX & ATX today can overlap on quality & watts with two exceptions: 1) ultra-short millisecond-type GPU spikes / transients beyond the rating (due to smaller capacitors on SFX models) and 2) availability of 850W+ SFX PSUs.
SFX are still quite a bit pricier & rarer (e.g., availability, region), though. Still, 650W from an SFX PSU = 650W from an ATX PSU, all other things equal. To actually compare SFX vs ATX: it'd have to be a model vs model comparison, as the overlap goes both ways: good SFX beats bad ATX, but good ATX beats bad SFX. The PSU's test results matter more than the form factor, which is an improvement from the earliest SFX days.