I've used both a ton....
The Slik is going to be more durable (shell and insulation), but the Montbell is hard to beat for comfort.
If you're a cold sleeper, neither are going as warm as their posted rating. So if you get a Montbell UL Spiral Down hugger with a 15degree rating (and you sleep cold), than you'll be good to 20-25 degrees. The Slik 20 will be good down to 25-30 in comparable conditions.....both are 5-10 degrees off when you sleep cold.
The UL series from Montbell have down insulation, so you can't get them wet, and as I mentioned above, the shell on the Montbell is somewhat fragile. The Shell on the Slik is made of Rhino skin and is as tough of an outer shell as I've used.
Like anything, it's going to be personal preference, so base your decision off of the feedback that you get on this thread, for your particular/intended use (from guys that have used them).
I know Rosinbag really likes the Montbell, so he'd be a good guy to comment on that bag.