Depends on your health. I rode the bike a lot before my hip replacement because walking hurt too much. Biking is good cardio and keeps the moving joints lubricated. Walking does the same but doesn't have the cardio benefits. Now (post hip replacement) that the weather has cooled enough I'll be doing both.
I use the exercise bike at the gym 4 times a week (45 minutes) and walk three times a week (60 min in the AM and 20 min after dinner). That regime combined with my HFLC lifestyle have produced incredible results: Goal achieved! Scale reflected 202.0 this morning down from 252.4 on Apr18. That's 50 lbs my fellow Sparkers. Tracking every calorie works! PS - I am 55 years old.
Bike! It's so satisfying to see all the beautiful sites while you're riding. You've got the breeze in your face (maybe not so good in the winter lol), and just a refreshing feeling all the way around. You'll build up strength in your legs and butt to start tackling some of those hills. Better yet, start with a simple paved path ride (you don't have to worry about cars), and migrate to off-road rides on greenways and such. It's wonderful!