Dominic

assuming you want the early g3a3 (OD furniture with holes in handguard)
both marui and classic army  have G3A3 s,
here are some basic differences between the two brand:

Marui : plastic receiver and frontset, cheap alloy metal parts, nice and stable internal parts

Classic Army : full metal frame and frontset, minor flaws like casting marks on the metal parts, rough quality internal parts, wrong markings on receiver..

the marui one is out of production for a while so your best chance is a second hand one. If you want a new one go for classic army. If you want a really nice G3A3, I would suggst the marui g3a3, (or a g3a4 / sg1 if you can't find one) and fit in authentic handguard, pins and stock..these can be seen once in a while at online mil. surplus shops..

Dominic

get the real stuff, only need slight modifications
price is reasonable too

Dominic

* of course it all depends on your budget, if you want a realistic one, modify with a marui as base, with authentic furniture and metal parts..
If you have lower requirements, just get the classic army one, it works fairly good after modifying and a little fittings..