I often get confused about which one to choose when I speak English. Please help me.
Ex) The thing that I was talking about is this one which is item number 3.
Ex) the thing that I was talking about is this one, and it is item number 3.
They look all the same to me and so I just use them without distinction. Can you let me know which one is better in which case especially in terms of speaking?
Both are correct. Neither is better than the other (at least in your sentence) and yes they pretty much have the same meaning.
In terms of speaking, I recommend you to use this sentence instead:
The thing that I was talking about is item number 3.
Shorter and simpler. If you and the person you're talking to are in the same place, you can just use your finger to point that item so you don't need to use "this one which is" or "this one and it's".
If you really want/need to use "this one", you can even say:
The thing that I was talking about is this one, item number 3.
