Which cardio is best? Please opine

To my readers who are knowledgeable in this area, I pose a question. Which of the following two cardio workouts are best?

1) 45 minutes on the treadmill, changing speed each minute. Walking fast at 3.5 mph, walking really fast at 4.0 mph, and running at 5.2 mph. This leaves me tired, gets me breathing hard, not much sweat, heartrate not too high, and my calf muscles get really tight in the front to where I have to stop and stretch them as my feet quit working

2) 45 minutes on the Precor EFX (eliptical type machine), using an alternating resistance program that changes about every three minutes. Doesn't make me too tired, but I notice that my heartrate is higher than when on the treadmill. Legs don't tighten, I sweat like a dog, and I usually get a nice runners high, i don't breathe as hard as I do when i'm on the treadmill.

Time is precious to me. It is hard to carve out 45 minutes for me to do this cardio, so i want to make the best use of my time. sometimes the decision is made for me as all of one type of cardio equipment is in use and I'll have to pick what's available.

Goals are to be able to run a 5K by October, and to of course lose wt, body fat, and get in shape. I figured that with a goal of running a 5K, it would make sense to use the treadmill, but I question this as I get a higher heartrate, more sweat, and a high with the eliptical.....

Thoughts? Let me know.