"could say" or "should say"

如果想話比佢聽句說話咁樣表達會好D, 我應該用"You should say" or "You could say" 會比較溫和而又可以帶出"其實基本上係要咁講冇其他選擇"

According to your Chinese, grammatically I should say expression 2 is the appropriate one.

you could say = what someone says is acceptable, and there is another way to express the same idea.

you should say = what someone says is not really correct or barely acceptable
and the correct or proper expression is :.....

Having said that, in workplace communication, you should take into consideration who you are speaking to:
if you speak to your peers or inferiors, you should always say: you should say.
but if you speak to your boss or superiors, it will be an offence to them. Instead of correcting their mistakes, use you could say instead.

我認為你應該用這種語氣表達出來會更好些==I thought that you might express your idea in a better mood.