In most situations, we simply say 'advice', as an uncountable noun: My advice is this,. Let me give you some advice What's your advice? Did he give you any advice? I asked her for advice. That's good advice. They gave us some bad advice If, for some reason, you really need to use 'advice' in the singular, to refer to one specific tip, you can say 'a piece of advice'. For example, 'Most of what he said was useless. In fact, he only gave me one decent piece of advice during the whole consultation'. However, it is rarely necessary to to say 'a piece of advice.' 'Advice', as an uncountable noun, serves English native speakers well in most sentences. We don't usually feel the need to refer to a specific 'piece of advice'. Meanwhile, many non-native speakers are overly obsessed with the 'piece of..' construction, simply because the equivalent word in their own language is singular. It's a case of using the words of a second language while retaining the concepts of one's first language. For example, a Spanish speaker will be determined to translate 'un consejo' as a singular countable word in English , or 'consejos' as a plural word. You really don't need to do this - 'advice' as an uncountable noun is all you need 90% of the time.

They both mean the same thing essentially. I'll show a few exanples: "My advice is to go to the store." "My piece of advice is to go to the store." Really, you can say either one whenever, it is not like you have to say one or the other in a certain situation.