Data within the game’s files provides a list of raw probabilities for Tokens. It is unknown if this is accurate, since the game’s server is ultimately what tells the client what reward it gets, but it also sends some odds along with it, so I’d advise a grain of salt, but they seem pretty accurate:
The quantity of Tokens you receive is based on the difficulty you play. A Difficulty 10 Boss Chest has a 70% chance to drop 1 Token and a 30% chance to drop 2 Tokens, making the expected average Tokens per chest over time 1.3. Here’s the full table for that.
Additionally, the rarity of Tokens you receive scales with Difficulty as well, so you’ll have a tough time collecting Common Tokens in Difficulty 12.
|Difficulty||% for C||% for R||% for UR||% for E||% for L||% for M|
Multiplying these together can give us an overall average Tokens per run, divided by rarity:
|Difficulty||# of C||# of R||# of UR||# of E||# of L||# of M|
Which mode is most appropriate for you is heavily based on your speed in battle, but we can see here that the expected yield of each rarity increases with difficulty, except for Common, where the decrease in probability of Commons in higher difficulties lowers yield.
TL;dr If you’re looking for Common Tokens, Difficulty 10 has the best odds, otherwise do Difficulty 12.
EDIT: Medals -> Tokens
EDIT2: All tables now account for the base drop rate of 1 Token per run.