Probably the biggest question is what's available when I get interested in a series. A good example - I just started in on BPRD, and the HC collections are nice and reasonably priced, especially for the quantity of content. I probably could have gotten the trades, but the HCs look nice and the price is about a wash. If I'd started on BPRD when there was only a couple of trades available, that's where I would have started.
There are series that I got in on the floppies, ones I got started because I found a cheap trade.
But for the giant, $75+ omnibuses, I just don't go in for that. I'd have to already love the series, and also not have any of the series in order to want to do that, which isn't a situation that pops up often.