As for hoods with the Elmar and Summicron lenses... I almost always use a hood, and usually the hood stays in its position when the lens is not being used. So I'm talking hood use on both lenses, not one or the other without hood in comparison. Its been a few years since I had my new Elmar but these are the things I remember.
If you use the provided screw in hood that comes with the Elmar and keep it on, when you collapse the lens it is not much of a 'compact lens', and I can't say its a very effective hood as hoods go. If you unscrew ( and screw back on ) the hood every time you also are collapsing the lens you're wasting a lot of time. You could use ( as I did ) the quicker clip-on 12585 hood which protects the front element better, but again its a matter of taking it off when collapsing and finding a place to put the hood.
The Summicron ( I'm using a 11817 or '69' version ) is fairly compact in itself, 42mm in length verses 38mm for the extended Elmar or basically the same size. The hood reverses and stores on the lens very quickly.
The new Elmar is on paper a 'better' lens maybe, less flare and sharper across the field as its stopped down to its optimal aperture. Its a modern lens with a very good performance no doubt. It had 'bite' with color film, but I would not call it a classic B+W film lens. What I've always liked about the 11817 is that it has a classic look with a bit more contrast, a nice balance, is very good in the close range, and stopped down is really excellent. After 30+ year of use I can say its super well made, like 'liquid' to focus ( I didn't like the focus of the new version Elmar ). It does not handle flare as well as the new Elmar ( better than the old one though ), but does have the stop advantage which for me is a very big plus in a Leica lens. And again basically the same size - unless you're walking around with your Elmar collapsed without a hood on.
I guess the question as far as compactness and the collapsable feature is, how often are you really going to collapse the Elmar lens for storage/transporting? For me it was not often.