It all depends on how often you log in and if you have the money handy.
If you dont log in often, hit the full stadium upgrade and let it complete.
If you log in daily, you can still do this, but you might not want to do it all in one go if you have other upgrades you are waiting for, reds and green packs...
If you have the money and dont care about other upgrades, do the full stadium upgrade instead of seats.
If you dont have much money, the seat upgrades are perfect. I am doing that one myself because I plan on buying a player and need the cashola. the full stadium upgrade costs $1.8M instead of the $600K for the seat upgrade allowing me over $1M more to spend on a player.