The difference in def might look big but you have to remember mages don't have high enough shielding to make much use of the def value.
According to tibiastats, the average damage reduction in full def you get from a shield with 26 def is around 22.
Now for a shield with 20 def the avg damage reduction would be 17. For those calcs i used shielding 37, which is already decently high.
As you can see, if you were to take only the def of these shields, the Shoulder Plate would only reduce 5 more points of damage in avg than the Lion Spellbook.
Now the actual damage reduction when you actually add in the % resistance (SP = shoulder plate and LS = lion spellbook):
- If you were to take 100 damage, SP reduces it to 76, LS reduces it to 80
- If you were to take 200 damage, SP reduces it to 174, LS reduces it to 177
- If you were to take 300 damage, SP reduces it to 272, LS reduces it to 274
- If you were to take 400 damage, SP reduces it to 370, LS reduces it to 371
- If you were to take 500 damage, SP and LS reduces it to 468
- If you were to take 600 damage, SP reduces it to 566, LS reduces it to 565
In conclusion, regarding only physical attacks and in average, Shoulder Plate is better if you're gonna take hits lower than 500 and Lion Spellbook is better if you're gonna take hits higher than 500.