Another bonus with slower upgrades, apart from those already listed, is that you get much greater build flexibility. You can do one small upgrade, then work on other areas of your facilities, like training or youth centre. If you do a larger upgrade, you have to wait even longer before you can upgrade other facilities. On top of this, you risk losing possession bonus in matches from not having a full stadium (depending on fanbase and other factors, of course). It's up to you, but doing the smaller upgrades first will cost the exact same amount of time and money as the biggest stadium upgrade.