I'd say part of it depends on if you see yourself getting more hens later on. If you'll always keep a small flock, that's one vote for a tractor. I have both a stationary coop and a tractor...and I can tell you that they will demolish the grass under the tractor if you don't move it consistently.
We also have the same problem...though we have a nice sloping backyard, and lots of tasty bugs, I can't let my girls free-range because there are hawks that literally live in the trees above our yard. I'd have to be standing there the entire time, and even then, hawks are bold (we have 12 hens, so it's a lot to keep track of!).
The votes for a tractor is that it's portable, you could always take if you move. On the other hand, to make it entirely secure, you would have to put hardware cloth along the bottom to completely seal it off from predators. Then your hens are always walking on wire. In my opinion, that's a rough surface for them to be on their whole lives, but hens have survived worse! My tractor is open in the bottom because they are not in it 24/7/365, so I'm not worried about critters getting to them.
What's nice about a stationary coop and run is that you don't have to worry about this if you build the run well - they'll have outdoor access all the time, with probably a sand bottom, and some sort of roof (maybe?) to keep out some of the rain. We did the whole bury-the-hardware-cloth-12-inches-deep thing, and that wouldn't be too hard for a smaller coop. As someone mentioned, it would be great to build a larger stationary coop in the event you even want to add one or two more hens as the years go on. Though I love my tractor, that's what I'd recommend! I think you might find you want to add a few more
