Here is a super short blog post, comparing metal vs resin brake pads for hydraulic disc brakes. To be specific, here are the L03A resin and L04C metal brake pads for Shimano road hydraulic disc brakes. The older model of the resin pad is L02A.
Most of the time, resin brake pads are good enough for daily usage, even in wet weather. However, if you need to ride in poor conditions, such as in muddy terrain, metal brake pads can last longer. For the pros and cons of metal vs resin brake pads, you can Google for the answers.
Metal brake pad on the left, resin on the right. The metal pads have much more metal content inside as seen from the reflectiveness.
The metal pad has a separate metal backing, which is riveted to the aluminium plate with cooling fins. I am not sure why it is made this way, instead of the bonded construction like the resin pad.
A pair of resin brake pads weigh 15 grams. These are the type with cooling fins.
A pair of metal brake pads weigh more, at 24 grams. These also have cooling fins.
Per pair, the weight difference is 9 grams. For both the front and rear wheels, the weight difference would be 18 grams. Not much, but it does count for something if you are tracking every gram. In that case, you should get the resin pad WITHOUT the cooling fins, which would be even lighter.