不一样，具体的不同如下

nothing serious为代词+形容词结构，中文意思是不严重；没什么大问题

Just a slight cold, nothing serious.有点伤风，不要紧。

not serious为副词+形容词结构，中文意思是不严肃，不认真

This all means that if you are serious about making more money, speak up now, on or just after May 5 - there will not be another window of opportunity for a long while.