wrong wrong wrong wrong wrong
do the seat upgrade, small, medium, larger and then upgrade the stadium. After 16 days, you will spend the EXACT same amount of money. It will take the EXACT same amount of days. But by upgrading seats first, you get two advantages.
First: you get to take advantage of the extra seats after each upgrade. Extra seats, mean more ticket sales which translates into more money.
Second: You spread the financial commitment out over 4 upgrades. So instead of one huge expense, you get 4 smaller expenses. Very important at early levels when cash is harder to come by.
Here are some pics to support that building seats first is better.
And to the OP, you will have a big jump in fans when you level up each season.