229 v. 226 and slide types
The primary difference is the one of dimensions. The P229 is a tad smaller and mag capacity drops accordingly.
But....early P226's had a stamped and pinned slide which made them a bit lighter than the current milled slide versions. So a milled slide P226 in 9mm, in my mind, is a bit heavier than it needs to be. In .40, the milled slide is a requirement with the higher energy cartridge. SIG P226's in .40 are a comparitively new item. When the .40 first came on the scene, SIG only offered the caliber in the milled slide P229.
So...if weight and dimension matter, the lightest compact high cap 9mm is the P228, now sort of rare, but out there if you watch. The lightest SIG full size 9mm is the P226.
If weight and size are what you are looking for, ie, the pistol must be .40 cal and the full size appeals,than a milled P226 is the answer.