Yes if you buy a SIG P226 in either 40S&W or 357SIG you can buy the other barrel and drop it right in. The mags of course are the same for both.
It comes down to your personal preference, feel and fit. Both are fantastic, reliable pistols, both also have quirks about them...eg Glock has a weird angle, Sigs have a thicker, roundish grip and higher center of bore...