I've gotten bored with the elliptical and figured it's time to switch things up anyways. I do a few 30 minute work outs during the week and am thinking of using the treadmill. Is it better to do 30 minutes at no incline but in the 3.7-4.0 MPH range or at an incline and 3.3-3.5 MPH? Or should I do some of each?

some of each, I recommend looking into HIIT (High Interval Intensity training)

Definitely do some of each during your workout. That's called high impact interval training (Google HIIT), and it super revs your metabolism and increases fat burning. Plus it's highly efficient, only needing 20-30 minutes total time. It's something you can do everyday.

You can do your workout something like this:

5 minute warm-up
Walk/run hard as you can for 1 minute
Walk at half the speed for 3 minutes
Then repeat the interval for about 3 to 4 sets
And finally, 5 - 10 minute cool-down

When you find that the higher speed interval isn't challenging enough, but you don't want to increase the speed, then increase the incline.

Georgia

I agree. Mixing it up is always the best. I am at the same as your current weight and have the same long term goal . I have just started doing the treadmill as well. I have just printed a plan to work off of that may help. It is C25K.com this is for Couch to 5K. It sounds like a great plan to get you up and running in a short amount of time. You should check it out.
Good luck!