Single issues for me, for the reasons you have given for liking them. Not only the letter columns are priceless and a great time capsule, but also the cliffhangers have some kind of impact. Plus, each cover and issue is usually specificaly centered around the particular piece of the story thats on the issue. And we get more covers, so more cover art, which, imo, is most of the time much superior to the trade covers
As far as digital, Im an old school guy so for me thats not a product at all, its a make believe thing that I cant own or hold. Its data. I cant say I own a painting if I dont have it at home to put on a desk and study and enjoy, but have a downloaded picture on a laptop or phone instead. The smell of print etc. Just an old geezers rant. In short, no digital for me