it really depends on how much cash, players u have on ur team
the basics is that the
bigger the stadium the cheaper the price u can sale ur ticket,
and therefore maintain a stabler Attendance income,
which in turn can use to afford higher total salary
the trick of building up ur stadium is keep the attendance @ 80% even at loss
try to finish the enlargement before price change
if its full then build the section that is not full
check ur coast standing, high seeding team have boast in the lower 2 sections
so if u are PO bound, build the lower 2 section first
if u are in the bottom, build the more expensive seats which affected less by the seeding
if u look at it long term, then its always good to balance it out
if u have penty of cash, and not planning to buy players any soon, u can always build the stadium first, its slower to get the result back, but it will eventually come back in about 10-20 home games