If you aren't going to collect the whole set, (remember the monster usually wins
) instead of choosing between high grade and rare backs...go for the Hall Of Famers!!
Get the nicest ones you can, because in 20-30-40 years, your potential buyers will know who Walter Johnson, Cy Young, Nap Lajoie and Ty Cobb are...but Chappy Charles may not be appealing to anyone no matter what the extinct tobacco brand he's advertising says...
And now I realize you already mentioned that you were looking at HoFers...I don't collect T206 because there are too many commons for my taste, and you cannot read the back to get any information about the players. (i grew up memorizing Topps cards)
So I revise my answer to higher grade common backs , of HOFers