Safety and comfort dictate that you buy a bike that fits you. A road bike should have about one inch clearance from the tube to your groin. A mountain bike should have about five inches. Also check the proper reach to the handlebars by placing your elbow at the tip of the saddle. Your fingertips should just touch as you grab the bars.
Test ride the bike.
In fact, test ride several bikes before you make a decision. Test a few seats, too. The seat should be flexible enough to absorb the impact of bumps and the pressure of your own pelvic bones. Yet, a seat should be firm, also. When you are ready, buy from a reputable bike shop.