If it's primary purpose is concealed carry, I'd go with the 738. I have a 709 which I love. But it's a little less discrete and a little tougher to carry since for me the 709 is too big for pocket carry. I have a P3AT and I carry that more often simply because it's easier and more flexible to carry due to it's smaller size and weight.
Too many differences to compare the two. I think you should decide what you want the gun for, in terms of conceal-ability. If your line of thought is the smaller the better, then you have the right gun. If you decide you want a larger round, than the 709 is the choice. Now if you are talking dollars, the TCP is less expensive. I have both and the wife likes the feel of the TCP and I like the 709, so that is another area to look at - how well does it fit your hand and how does it feel. Both are great guns and you can't go wrong with either one. If I were you, and since you already have the TCP, keep it and start looking for a 709. Best of both worlds, in my opinion.
Im going to have to agree with the other TCP oweners here I like it because I can toss it in the knife pocket of jeans and it stays there and is covered by a T-shirt, however if you want something with a larger caliber then I guess the 709 is your best bet. I do agree with Wino tho if your thinking about the 709 id just get the 740 slim