In the short 4-5 years I've been playing miniature games. These are the Things I've noticed.
I haven't worked in Resin at all, but the models I've seen have far better detail than any metal or plastic.
1- Weight- This is a good and bad as I see it. For shipping it could be a nightmare if you are shipping a lot of mini's in metal rather than plastic, but a metal mini has a feel to it that plastics just don't have.
2-Painting- I haven't notice a real difference in the way they hold paint, although I have had a lot of my metal models chip, I haven't handled my plastics enough for me to test them. But this topic brings me to my next point.
3-Cleaning- I recently decided to repaint my entire Khador Army and had to clean them. The metals came clean soooooo much easier than the plastics. I can see from this why people say the plastics hold paint better.
4-Detail- In this category I've noticed that metal mini's have a similar detail scale. I think that detail can depend on the company who makes the mini's, but in general, most metal used to make the models has a similar quality, texture and feel. Plastics on the other hand come in a wide variety. The difference in Wyrd's Plastics compared to GW's or PP's is very different. I haven't worked with GW's, but their plastics look like high quality. PP's on the other hand have some flaws and I think that this comes down to the type of plastic used. Its softer and therefore loses some detail when things get really small. So far I have painted 3 different Man-O-War Shocktrooper units, 1 metal, 2 plastic. While the outstanding larger details are better in the plastics, like spikes, heads, weapons and other things that stand out easily. The lower priority areas like feet, boiler grills and sometimes hands get a little fuzzy. I have yet to see one of my Wyrd plastics look bad.
That's my 2 cents.