is one better than the other (Q re walking routine)

aadutton

so, first of all, let's put aside the cardio/strength training debate. i do lift and do other cardio; this question is just about my walking routine.

i try to do an hour on the treadmill x 4 days per week. i'm not running (much) yet, i try to average a 3.5 mph pace at a 3% incline.

sometimes i walk at 3.5 mph for the entire hour. obviously i go 3.5 miles if i do this. sometimes i do more of an "interval" routine that warms up at 2.5 for a couple minutes, then 3, then alternates between 3.5 and 4 for the majority of the hour, with a cool down similar to the warm up. as it turns out, i go 3.5 miles doing this as well.

so, in both cases i'm averaging 3.5 mph. obviously my heart rate is varying more with the second routine. is it "better" exercise?