The price difference you are seeing is that Umarex ones are licensed by H&K and are also made by Ares, so they are authentic in terms of detail and generally decent quality, although where Umarex is concerned, there are several differently priced Umarex G36 variants, some electric blowback, others not, although they are all fairly decent quality. Most of them have a quick change spring feature on the gearbox. Having said that, the Jing Gong G36 is a favourite at airsoft sites for use as a hire gun, because it is pretty robust and reliable, but of course it is not licensed by H&K, nor made by Ares, which is one of the reasons it is cheaper than the Umarex one.
As Monty mentions, the SRC G36 is good too, so give that a look, but be sure to note that the SRC G36's magazine is well known for being problematic, suffering from feed jams. No big deal though, because other brands of G36 mags do fit the SRC G36, so not a major issue. And of course you will be wanting more mags anyway, to take advantage of the funky jungle clip system the G36 mags have. On the subject of which, most airsoft G36 mags will work in other airsoft G36s, with the notable exception of Well ones, which are very different from other G36 mags in terms of where they feed. If you want low cap G36 mags, ironically, the cheap Double Eagle compatible ones are worth checking out (they take 30 rounds).
