I love working tethered with the Leaf, it can only be done via Leaf Capture for the moment though I have heard rumours it will eventually also be integrated in Raw Developer.
It is fast, images are on screen fairly fast but the best part is that it works flawlessly. Janking out the firewire cable and putting it back in, LC11 doesn't care and just continues where it has left of. Images come up on screen as big as your screen allows. I use 2 screens, 1 21" for the previews another 21" for the tools, etc..
There are people that shoot into a folder that is being used in Lightroom and immediately imported. I have no experience using it that way. I shot into a folder that I had open in CS3 Bridge. From there I made adjustments and ran it through the imageprocessor with actions if needed. The files end up in a folder somewhere, there is no restriction in you running another piece of software picking up the files where LC11 puts them. I would love to see a REAL 'hot folder' solution while shooting tethered sofar none of the backs I use provide this. There are several options to get to this but all of them involve scripting or other forms of automation software (I have not yet been annoyed enough to actually make something like this, I guess I should )
I cannot truly compare shooting tethered with the Leaf with shooting tethered in C1, I can just say that shooting tethered with a Leaf has never disappointed me though I still have some stuff on my wish list.
LC11 is very stable now, you can process large amounts of files (at least I can) without crashes or other nasty stuff. Only the processing to tiff of jpg with LC11 is slooooowww........ You are better of using RD, IMO. However the stability is great, I closed the lid of my MBP more than once when LC11 was still processing, it just continued when I opened the MBP again!
Get it over with and go for either one, I am sure none of them will let you down From all of your considerations both backs appear to be pretty equal in pros&cons for your typical use. If you are truly in need of an MFDB you are now wasting time you could have been shooting with either one of them.