They both look very sexy, but from the way fashion trends go, the first one will stay in fashion longer than the second one. The second one has a lingerie-ish look to it, which is predicted to only last a few more seasons before it goes into the historical fashion trunk.
The bottoms of the first one makes the model look like she's wearing a thong with other random stripes across it though, lol. I like the solid black bottom of the second one more than the half black, half patterned bottoms of the first one... But if I had a choice, definitely the first one.
Also, if you want to look bold and sexy I would suggest a white bikini. Most women buy black bikinis because it slims down their appearance, but white gives a bolder look and shows off a nice killer tan on the beach