I am in a somewhat same but different situation.
I have mainly stuck to collecting 52 Topps cards but I have always wanted and coveted a 56 Topps Mantle as well.
Right now I can get a newly slabbed PSA 4 for a very fair/decent price but I can't make up my mind to purchase it or put my money towards more, specifically high numbered 52 Topps cards?
I still need many lower numbered cards but I thought if I am willing to spend $500 or so on one card that I love, should I maybe spend that on one or a couple high number 52 Topps cards?
I know the decision is mine only, but for the life of me I can't figure out what to do!