SSP= stock service guns. G17/34, M&P, XDM. Division is ruled by 9mm as well as ESP, but for the most part the gun needs to be stock
ESP= trick your SSP gun out. lightened trigger, magwell as long as it fits in the box, basically w/e you want within the limits of the rulebook
CDP=.45's! It can be a 1911 or G21,M&P45,etc
My best advice for getting into competition is "buy yourself a Glock". Im shooting a Wilson CQB in CDP/Single Stack right now. Next year, I know for sure 1911's will not be used in my rotation because Glock's and M&P's are simply faster! The big advantage comes during the reload. If you are more worried about having fun while shooting a 1911, then by all means, go with the Sig or anything else! Id also recommend you take a hard look at STI's lineup before you purchase anything from Sig Sauer, but if you want to be the most competitive, I think the G21 and M&P are going to be the 2 fastest guns in CDP.