For a primary carry I would go with the 9mm, and get a .380acp for a BUG or another carry option at a later date if finances don't allow for both at this time. If you had to choose only one I would recommend the 9mm because as others have stated the cost of 9mm ammo is about half the price of the 380acp ammo so you can practice more with it as 9mm is more readily available. And not to mention it packs more of a punch, seems like a no brainer. I guess it comes down to how your planning on carrying, IWB, OWB, pocket, ankle, shoulder, etc. If you prefer to pocket carry or don't like the idea of a heavy gun on your waist the 380 would be the way to go. But for me I don't mind carrying a 21oz M&P compact 40 inside my waistband. I would suggest the M&P9c it's a great gun.
I carry my primary S&W M&P 40c which I've converted to 9mm for the range, and carry a S&W Bodyguard 380 as a back up or rarely I will carry the 380 by itself.
Good luck and welcome to the wonderful world of guns, and be warned it's additive! I'm sure you'll probably end up having something in every caliber eventually if your like me!