Champion shards give you essence based on how much they cost in store. A 6,300 champion shard will give you way more essence than a 450 champion shard. If you're saving up, I wouldn't waste your IP to get your champion to Level 6 Mastery with tokens, unless they're your absolute favorite, and you want everyone to see that you're good at them, hahah.
(I did that with Taliyah. I really wanted to get her to Level 7, so I binge bought a ton of Tokens as soon as I got the IP for them.)
My suggestion? Just wait for your earned Hextech Chests to drop Champion shards you don't need instead, and pray for those 6,300 champion shards. It might take awhile, but it's better if you still need champions to be unlocked like Thresh! Once you save up some IP and don't really want to unlock any champions anymore, then you can decide if you want to purchase those tokens.
Edit:
I have 5 sets of Level 7 Mastery shards, but I don't waste my IP on tokens for them. I just wait for chests. The reason why is because of that: I might get a champion shard that only gives me 100 Blue Essence because they're a cheaper champion. Spending 1,700 IP for a chance of a low blue essence shard is rough.
I'm still unlocking champions so I just save up for them and use shards I don't need from Hextech to slowly level up my champs!
