The SP is about the same size as the P229 just lighter due to the polymer frame. Max mag capacity is 15 (depending on where you live). With the P228/229 you can use the higher capacity P226 mags as they will fit and work. I've got several 17 round P226 mags for my P228 and used the "X-Grip" plate to cover the gap at the base. Fully functional and cosmetically cleaner than having the naked mag extension stick out.
Since you work for a private firm, you may want to check into the policy before going the route of the P226 mag capacity in the P229 you currently have. If 15 is the max you can carry, then either you P229 or the SP will be fine. Comes down to weight and personal preference. The SP has one of THE most underrated triggers ever. After it breaks in, it only get better.