The great majority of my life I kept trying and trying and trying to read Lucretius and never made any headway, so I never suggest that anyone start with it at the beginning. Much better is to get a fundamental grounding in the philosophy like DeWitt gives, and then you are equipped to understand what you are hearing.
I know I am not the sharpest knife in the drawer, so I am sure some people can pick up Lucretius and immediately get into it. For me it was largely gibberish until I heard Charlton Griffin read it to me, and then (if I recall - sort of simultaneously) read DeWitt.
Having someone read it who knows where to pause, to stop, to emphasize, and how to change tone with the subject -- that makes SO much difference!
And then on top of the reading, being given a framework for putting it all together - which is what DeWitt does -- for me that was the key.
Now that I have finally slogged through it I think I see that the best thing - generally - to recommend is to start reading DeWitt first, and then pick up Lucretius after you've at least read chapter one of Dewitt, and preferably the whole thing.