I mostly buy digital when they come out. For older series, I wait for Comixology sales and try to pick them up for $0.99.
I do buy print for single issues if the panel layout is not conducive for digital reading like Swamp Thing.
If I plan on buying a long run, I will look into the TPBs so to make it cheaper. I have the Brubaker Cap omnibuses(omnibi?), Busiek's entire Avengers run, and Moore's complete Swamp Thing run.
I like the mobility of having just about my entire collection with me wherever I go, not having to store a lot of books, and I don't think many (if any) of these single issues will ever rise in value so digital is the way for me.
X-O Manowar from Valiant. It is a great story. It's also on sale for $0.99 an issue on Comixology. The main character reads like an amalgam of Thor, Captain America, and Iron Man. He has the personality of Thor, he is a man out of time like Cap, and he has an advanced suit of armor like Iron Man. Big fan so far.