Can we have a mana+MR item?
For everyone that isn't
there's no significant difference between itemizing for MP/5 to have enough mana to cast things or itemizing for mana to have enough mana to cast things.
If Mana pools were so low you'd run OoM from casting 1 skill, or if you had no base MP/5 those 2 stats would have very different strengths and reasons to itemize for: but currently you have enough mana in bases to QWERQWER before running OoM and base MP/5 is high enough to throw out 1 Q every 30 seconds/minion wave to CS+harass+wave clear near indefinitely, making it extremely unlikely you'll be able to stay out in the field long enough to see any difference 1 way or the other concerning mana or MP/5 before you wind up loosing enough HP you need to B.
There's really no reason to have an MR+Mana item when
already exist to fullfill the exact same purpose.