don't learn SNGs first
cash is harder than SNGs

if you only learn SNGs you won't be able to play cash
in cash you play against full stacks and short stacks
in SNGs everyone has a short stack

if you have a deep stack you get to make more river decisions, which is the street that you have the most information on
in SNGs you'll never learn river or deep stack play since it's a shove preflop or on the flop