At the entry level, there is no real difference. You are going to have to learn the rules and requirements from scratch anyhow. You can get into Divisions, and Categories, and guns and gear and philosophies later.
The way to get involved is to get involved. Use the www site club locators, ask at local gun shops and ranges. Find the closest place that shoots either and GO.
Check
www.uspsa.org for club locations and general information about IPSC in the United States, the IPSC.org site gets you the parent organization and USPSA is our regional HQ.
All you must have is a handgun of 9mm/.38 Spl or larger caliber, a holster, several magazines or speedloaders, a carrier for them, ammo, eye and ear protection. IDPA calls for concealed carry but will often waive the requirement for Novices, until they are comfortable and safe making a draw from under jacket or vest.
You do not have to study or practice to get knowledgeable enough or good enough to shoot. GO! The clubs know they need new shooters and will accomodate you. Just be ready to pay attention, especially to the safety requirements. Some have formal introductions, all will give new shooters extra attention. Your main problem will be absorbing all the information you will be offered and picking one experienced shooter as a mentor to show you the ropes. You can of course take formal training, but few people do.