Having just completed Mark, firstly for myself and secondly with my kids, I would answer - both.
I primarily went with Mark, when I had questions about a parable or saying that was difficult it was useful to look up the parallel passages in the other Gospels, using them as "commentaries" in a way. When it came to the Last Supper it was interesting to read the other gospels and fill in gaps.
I realised that Judas left before the Lords Supper!
Peter gained access to the courtyard by John (who was at least known by Caiaphas/Annas, if not a personal friend) coming back to the gatehouse and personally vouching for him.
Despite accusing Jesus of saying He would destroy the temple and build it again in three days, they remembered the original context and petitioned Pilate for a roman guard against the possibility He rose on the third day! (Luke)
So my advice would be to stick to one gospel, but if it takes you to the other gospels so be it.
