Hardy +1 on first two posts. Following their advice will assure you of many pleasant riding hours.
No experience with bikes in question. Suggest getting best you are comfortable with financially and physically. Then get custom fit not just eye ball by LBS. Can do custom fit after a few months riding and then follow up in a year or so as your body will change.
As for goals, I'm 69 and started six years ago. Average ride this year is 49 and speed recently about 17.5. Average speed past year was 15 and 14 before. Oh yes, dropped 40 pounds since started riding. Like many others riding can change your life by getting and keeping in good shape. The adrenalin you feel after riding is wonderful.
Use this forum and Mr. Google to learn the many fascinating aspects of this sport or passion like how to shift, how to pedal, steer, turn corners, draft...