1878 saw a doubling of the Proof mintage from the previous year, indicating a significant increase on collector demand. This was the beginning of a rise in Proof Cent sales that peaked in 1886. As such, the 1878 is a fairly common coin in Proof condition, especially in PR63 and PR64. Surprisingly, the PCGS Population Report shows only slightly more 1878 Proofs than 1877 Proofs, and we all know the huge price differential between the two.
Red-Brown seems to be the most common format, trailed by Red, then Brown, then Cameo. No Deep Cameo examples have been identified yet.