Performance of physical cores VS hyper-threading?

Hello. I'm using two Xeon E5-2680 v4, ASUS z10pe-d16 ws M/B(Dual CPU M/B). As you know that there're physical 28 cores and 56 logical ones.

I hope you give me your suggestion whether I should better use 28 physical cores or 56 logical ones for the parallel computing? Which one has the better performance for the parallel computing? Are they the same performance or not? Then why? Usually, I used 55 logical cores but recently, someone said to me that it is not better than that of physical 28 cores.

I'm looking forward to hearing from you. Many thanks indeed.