Sansar is nice, but not directly comparable to SL (it reminds me more of would-be SL competitor Blue Mars). There is no single world; instead, it's a platform for the creation of stand-alone "experiences" (which can include teleport gates to other experiences, by all means). The building tools are also limited. Judging by my (so far also limited) experimentation, you end up working alone, mainly in something like Blender or Maya, then you upload and position your meshes in Sansar. In SL, you can collaborate on builds in real time.

An interesting open source alternative in the social VR space is Phil Rosedale's High Fidelity: https://highfidelity.com/

But personally, I am currently infatuated with the much more light-weight Rec Room: https://www.againstgrav.com/rec-room/