In most cases Second Life and OpenSimulator provide the same basic features but there are some differences.
Second Life makes it easier to socialise with other members as it is easier to move between regions than it can be on OpenSimulator. Any objects you own will also be available to be used in any region.
OpenSimulator has the ability to back up the created world, part of the world or single objects. This is not provided by Second Life so creations on Second Life can be lost.
Because of these differences there are reasons to choose each platform. to me Second Life seems to be the choice if you are looking for the socialising aspect of a virtual world but if you are looking solely for the building and development aspect of a virtual world then OpenSimulation may be the choice as you are able to back up the world or any objects created.