Plastic guns are not always lighter.
Plastic bodies guns can be just a heavy as metal ones. The receiver weight will only account for 100g or so between a decent plastic body and a generic chinese metal body found on sub £200-250 guns. Choice of stock, rail and barrel-length is a bigger consideration IMO for a gun if you want something light weight.
Soooo......here are 5 of mine/my kids *roughly* similar-sized guns, stripped of their accoutrements such as vert grips, sights and lights etc:
View attachment 92819
4 of these are 'plastic' bodied guns. One is 'full-metal'. And the picture above is in weight order.
So - heaviest to lightest (left to right):
ICS CXP-16 (Plastic Body - Metal Rail) - 2473g
BO Dynamics LT595 (Plastic Body - Plastic Rail) - 2327g
Specna Edge E12 (Metal Body - Metal Rail) - 2054g
KWA EVE-4 (Plastic Body - Plastic Rail) - 1946g
G&G CM16 SRS (Plastic Body - Metal Rail) - 1845g
So there is a 628g/1.3lb difference from the heaviest to lightest. There is only a 200g difference between the Specna Edge and and the G&G CM16 SRS.
This is a very un-scientific review of their weights - some stocks are heavier than others (the PTS EPS is a bit of a tank, but amazeballs), some gearboxes are heavier than others (ICS and KWA) etc. but you get the idea.
So - my advice is to try and get to a local Airsoft shop (if that's feasible) and pick up as many as you can to get a feel of them. If you can't to a shop - ask other players at your next skirmish about their guns - many will be happy to let you fondle their weapons (boom boom).