Things are a little complicated, so bear with me.
While @itzhak1969 is partly right in that upsampling (converting from one format to another) adds no data, that does not mean it did not sound better.
In the old days DACs just performed much better at high resolutions, and it was not dependent on the original sampling rate. Over the past 15 years the gap between how well they played Redbook (44.1 kHz/16 bits) and high resolution (96kHz / 24 or higher) has really narrowed, and even budget DACs sound really good at Redbook playback. It is my suspicion, without evidence, that this is related to hyper accurate, cheap clocks becoming available, as well as better circuits to manage jitter on input.
Between FLAC and WAV I would say, try both. FLAC takes less space, but there are those who used to claim that FLAC playback was more prone to jitter due to the decompression involved.
The most important thing though is what you can hear. We can talk hypotheticals until the sun burns out, but if you can’t hear a difference, it doesn’t matter.