Did Jesus tell his disciples everything, or hold some things back?

The two sayings answer different questions. In John 15:15 Jesus is explaining why he now calls the disciples friends rather than servants: a servant is told what to do without being let into his master’s plans, but Jesus had opened his mind to them. He had told them what the Father gave him to make known: who he was, why he came, and that he would die, rise and return to the Father. Gill notes that ‘all things’ here does not mean all that he knew as God, but all that the Father had given him to reveal for their salvation. Jesus says the same in his prayer: “I have given unto them the words which thou gavest me” (John 17:8).

John 16:12 adds that there was more to understand, which they could not yet take in: “ye cannot bear them now”. That fuller understanding would come through the Spirit, who would “guide you into all truth” (16:13), would “teach you all things, and bring all things to your remembrance, whatsoever I have said unto you” (14:26), and would take what is Christ’s and show it to them (16:14). So the revelation was given in substance by Jesus and was still to be unfolded by the Spirit. A teacher can truly say ‘I have taught you all I was sent to teach’ and also ‘there is much more you will understand later’.

Spoken as already done

Augustine suggested that in John 15:15 Jesus speaks of what he would make known through the Spirit as though it were already done, because it was certain, in the way the prophets speak of future events in the past tense. On this reading too the two verses fit.