I have an all-Dyneema SCB and an all-Cordura MCB. I started off with the SCB and it was my daily bag for two weeks straight, holding wallet, pens, keys, ID. The O-rings (or are they D-rings?) are a huge plus for me. I am a person who misplaces keys often, and though I already had my keys on a leather strap with a hook, my previous bags didn't have O-rings. I did not think it could get any better, and I really liked the way I can grab the base of the SCB and hold it against me in crowds for security.
I decided to try a Cordura MCB because I figured the additional heft would be useful in a larger bag. The Dyneema crumples fairly easily, which is what I want in a small bag (actually in most bags). My first few days with the MCB, I thought I had made a mistake, as my usual contents bounced around a little and I had a lot of empty space at the top of the bag.
Well, it turns out that the MCB showed its true strength as I began accumulating items as the day went by today and the weather changed. In went some cards, a small paperback book, and then a scarf. An extra pouch found its way in with some electronic cords. There still is room to spare. I can just about grab the base of the bag, quite easily if I tighten up the strap.
To summarize, if you are thinking about these bags, each one has its own uses. I can now see why people collect all sizes of the cafe bags. I am holding off on an LCB because it just looks like too much bag for me, but you never know. Plus I am very intrigued by the fact that a take-out pizza can fit in the LCB!