I bought the 2.8 70-200 USM years ago, and I absolutely love it. Price was a factor, but I can't say that there's any difference in build quality enough to make a difference, and if there's a difference, it doesn't seem to show in the results.
Solid lens and that's the reason they've sold so many of them to those of us who don't make a living with the lens.
And mine all but lived at dusty baseball fields for 8 years, and compared to gyms (wrestling), and grass fields (football), these dusty baseball fields are a good test.
And mine, like you work with kids, has been the starter camera for any number of "team kids" (player's brothers and sisters and parents), and even more scouts. All I have left for photography is to pass-along my love for it. WHile some just loved holding a big rig (compared to some), and to shoot "like motor drive" (I'm dating myself!!), some learned to love it. Many went on to do it in school, but all knew they could come ask for the camera and 999 times out of a 1,000 I'd hand it over and let them go discover. The beauty of digital!!!
Don't get me wrong, I LOVE film, but it's time to let go. My darkroom gear is all but gone, and that's OK. But I still love the memories of plate film (in the periscope), shooting 2-and-a-quarter, smelling like fix, and knowing what DoF is and how circles of confusion make if happen. I still teach them all the latter, and most get it, but more importantly they learn how to use it to their advantage. LOL, much like the AP v SP "sports pics" debate that'll never be won... It's all good as long as you're in the debate... And it's even more fun to debate B&W over color, nobody loses. LOL, sepia!! But hell, I'm finding that unless it's a big deal (and there are fewer of those coming by these days), I'm loving the portability of my Samsung S5 "camera"! LOL, I'm going to hell!!!
And I meant to ask and can't remember if I did, but did you ever buy the 20Da? You being into astronomy, I was just wondering..