There are several factors that help me choose which collection to pick up.
For modern books, there are some that I don't wanna wait too long to read, so I pickup whatever collections comes first, be it premier HCs, OHCs or even paperbacks (though usually, if the TPB comes first it's likely that we'll never see an HC, so that makes the choice a bit limited). For other modern books, I usually wait for the biggest collections like Omnibi or OHCs to become available; this gives me time to read some opinions and possibly one or two issues before deciding if I really do want that material or not.

Concerning classic material: I have lots of essentials and showcases that I bought a few years ago. Masterworks and Archives were too further down the line for me to pickup: too many volumes to hunt for, and living in Italy it was just too difficult to chase them all (also because online shopping was in its infancy at the time); also, Omnibi and softcover MWs did not exist yet. So I went for Essentials (and later Showcases), with only a couple exceptions for shorter series. 
With the rise of Omnibi, which I could pre-order and get as they came out as opposed to bore myself to death in long and fruitless hunts, I started to "upgrade" and re-buy the classic series that I liked the most at my first read in B/W.

Nowadays, I tend to wait for the Omnibi for classic material, though I still buy essentials/showcases if the material strikes my interest, but not so much that I want to spend more to have it in color. Of if it comes from artists I actually prefer in B/W (like Joe Kubert, for instance).

If there is no omnibus in sight but a new MW line is coming out and I'm interested, I will consider that option as well (I bought the Iron Fist MW when it came out).