League record is the key for me, it's a consistent format to judge a player especially against their peers. Cup games have a number of variables that while of course we want lots of cup goals they don't tell the story that league goals do.
As an example 10 goals in 4 cup games sounds great. But those games were against Kiddy, Walsall, South Shields and Southend.
2 goals in 12 cup games sounds cack especially in comparison, but maybe there were a bunch of weaker line ups, sub appearances, the games were all against elite sides etc. You don't get that range of variables in a league season year after year.
Exhibit A is Simon Stainrod. from memory he got 11 cup goals one season but only 1 was against a top flight side and that was Oxford.