in short, Urethane is much more durable but is heavy and can warp in extreme heat.
authentic Fiberglass kits are usually lighter, but are more prone to cracking.
fiberglass is cheaper to paint. Urethane is more expensive to paint, and if not done right the paint usually comes off in 'patches' instead of flakes (fiberglass).
replica Fiberglass kits are usually heavier than Urethane and are still prone to cracking. they also almost always need to be slightly modified for correct fitment. i'm not a fan of replica bodykits, but you will save a lot of $$ so it's your choice. currently, ALL replica body kits for the celica are made in Fiberglass--none in Urethane.