4 months ago
It depends. If one were to be successful with game tasks or did the most high earning tasks on offer walls, probably the ones involving mobile games, one would definitely get the most of soul gems.
But even then, doing the tasks would require some time and work, not to mention the offer wall tasks might sometimes be a bit unreliable with their rewards. Also, I get the impression the mobile game tasks must be completed successfully within a certain number of days, usually a week or so. Failure to do so means you'll get nothing, so I advice thinking carefully and checking your schedule before trying any of those.