So I've been mixing up my cardio between HIIT days, non-HIIT intervals and usually one day a week of straight running, but this question relates more to the non-HIIT intervals (on a treadmill).

When I started doing this 3 weeks ago (new shoes are AWESOME) I was doing 2 minute work intervals at 6mph and 2 minute rest intervals at 3mph. I'm up to 7.5mph on the work intervals (going to try 8mph work tomorrow) but haven't touched my rest speed. I know what I do should be based on my goals but I'm kind of torn as to whether I want to increase my work speed, increase my work time (or decrease my rest time) or increase my rest speed.

My original goal was to get to 9mph on the work speed and I'm still pushing for that, but lately I've started considering the alternatives; any thoughts?

If you want to burn fat, you want to have more rest time with high speeds between rest. If you want endurance, you stay at a jogging pace, maybe 6mph? You can also put it high, like 9mph and see how long you can stay there and when you're gasping just rest for a while, so you get the best of both.