Don't quote me on this, but I believe that there's approximately an 8% increase in the Blue essence you get, by investing you IP in champion shard tokens, and then disenchanting those shards into blue essence. (Once the 6.5x conversion kicks in of coarse, but you would want to buy and disenchant those shards before preseason hits. )
Personally, I've been getting the champion shards anyways because I have a few champions that I still need to unlock. (Based on PBE values, I'm guessing enchant cost is about 60% of what the permanent would normally cost.) So for the most part, I've been buying champ shards trying to get new champions, and disenchanting the shards for the ones I already own.