Is there a better metric than Market Cap or is this the only metric that is used to measure the actual value of BTC ?
Not to my knowledge, or at least not a commonly adopted standard. Anyone got any ideas?
Trading volumes are a by far better metric
Indeed, we don't know how much of the reported trading volume is real as in the first half of 2017 (and before that) most of Bitcoin trading volume was fake due to Chinese exchanges showing simply insane figures (like the total supply of bitcoins changing hands a few times a day)
However, if they are pretty close to real, which seems to be the case now, it is in fact a useful metric as it represents the real activity of traders and their interest in a particular coin (Bitcoin is still the preferred choice, though). And trading volumes is not the only pebble on the beach anyway (as there are many others)