Both the Siberian and the slipped buntline are great. I alternate between the two depending on my mood.
In my experience, a Siberian will not slip when there is tension in the standing end, and the Siberian doesn't even have to be dressed correctly while it prevents slippage. On the other hand, the slipped buntline does hold tighter than the Siberian once there is no longer tension on the standing end. This issue is irrelevant for everything I've done so far, and the advantage here may go to the Siberian depending on the application. I currently have a few trucker's hitches, anchored with Siberian hitches, supporting trees in my backyard. After many strong winds in all directions, the Siberian hitches are holding stronger than when I tied them.
For me, a Siberian has some advantages over a slipped buntline:
-I can tie a Siberian if the object (e.g., pole) is 20 feet up there and I'm way down here on the ground. For example, I can tie the Siberian down here and pull the standing end so that the Siberian holds tight against the object. The Siberian is basically a slip loop around the object. Some Youtube videos display how this feature works. On the other hand, in my experience, a slipped buntline needs to be worked with both hands all the way up to the object. The slipped buntline is basically a slipped clove hitch around the standing end.
-The Siberian is easier to tie, and I can easily tie the Siberian with thick gloves on.