I have a stock Sphinx SDP, with a 17 lb. CZ compact hammer spring installed. (That takes about two minutes to do -- its easy.)
I picked up a Cajun Gun Works Spring Kit for the SDP, but I haven't bothered to install those springs, yet. I may, eventually. The lighter hammer spring sort of fixed things for me. The factory DA and SA triggers are heavy, but very, very smooth. A bit too heavy for me -- hence the CZ compact hammer spring.
I also have a low-mileage SIG P228r that has been worked over by Gray Guns, including the Gray Guns Reduced Reset Comprehensive Duty Package which now runs about $325 (or so) plus shipping there. (That work, plus Heinie Straight 8 Sights) was done before I bought it.) It's a very nice shooting gun.
Note: I've had several SIG P226s over the years (including a P-226 X-Five in .40), for some reason, they don't work for me. I like the P228 and P220 better.
In most respects, the Sphinx and tuned P228 are very similar. Both are a pleasure to shoot, and both are VERY accurate. Their triggers are similar, with the SIG (after GG magic) being a little lighter, but no smoother. A stock P228 (or M11a1) (or P226) would not be as nice as either of these guns. My SDP is stock, the P228 is far from it.
One advantage of the SIG over the Sphinx is after-market support, but with the Sphinx hammer spring change there may be little need for after-market support -- maybe different sights, down the road, or finding a holster that you like. There are holsters available for the SDP.
SIG mags by Mec-Gar are cheaper than Sphinx mags, but the Sphinx SDP can use Canik Stingray-C 9mm 15rd Magazine that are available for as little as $25 each. I got a couple for my SDP from J&G Sales. (You may have to trim a little plastic off the top of the mag's base plate for it to fit the grip properly, but there plenty of plastic and no risk of making the baseplate inoperable. These mags also work in the CZ P-07.)
I like both guns, and in terms of dollars, I think the Sphinx might be a little better deal if you can buy one without paying full, high-end of retail. If I had to give one of them up, tomorrow, I don't know which one it would be.