| My guess is this: the hard fast bike ride is more stressful than the fast walk, you probably achieve a higher percentage of your max heart rate and when that happens, my observation of my own self shows that particularly intense exercise of less than an hour in duration will cause my blood sugar to rise, because the body responds by releasing glycogen and turning it into glucose for your body to use.
Better is as always a relative term, depending on what you intend to achieve with the exercise.
Overall, you probably burn more calories with the bike rides, which will help you towards your goal with the weight loss. On the other hand, the hard fast bike rides will tend to spike your blood sugars a bit higher than the walks. On the other hand, you may find over time and observation that the hard fast bike rides will lead to increased insulin sensitivity of longer duration than the walks. Or maybe not, it's individually variant and the best you can do as always is take notes.
Personally I think, the biggest thing is do what you enjoy doing, so long as you're not putting yourself at risk in some way. | 
