Swaro SLC edges it...
I received a pair of Swarovski 10x42 SLC HDs for my 40th birthday recently am completely blown away by them.
Did a side-by-side against a friends 10x42 HD ultravids on the first day in fading light, it was obvious that both were outstanding binoculars, but both of us agreed that the SLCs just edged it in terms of image brightness, clarity and sharpness, particularly in low light conditions.
I personally love the SLC's compact, rugged design too -- I find it much better for me than the elongated, double-hinged EL Swarovision. I like the Leica design as well -- but for me the SLC wins here too. They're perfectly balanced, the new magnesium body means they're lighter than previous models and are a joy to use, even for extended periods.
When looking at high end bins I'd pretty much whittled it down to Leica Ultravid HD over Swarovski ELs -- largely because of the handling and form factor. In the meantime the SLC HD was released, and changed the game somewhat.
My wife took advice from birding friends, local wildlife professionals and dealers before ultimately opting for the SLC HD.
I'm more than delighted with her choice... absolutely amazing bins! I have a
review of the Swarovski SLC 10x42 HD up on my blog, if anyone's interested.
In terms of making a choice both are superb binoculars. If you get a chance try and use them side by side for a while, even if it is only for five minutes outside a store. When you're shopping for optics at this level a lot of it comes down to personal preference.