Yup, there are differences between GHKs and WEs, although they do often make similar replicas of guns, so you could be forgiven for thinking that it might simply be 'badge engineering', which does go on in the airsoft world, but not in the case of GHK and WE who are different manufacturers. Both are well respected and make decent stuff, so it is often down to subtle differences between their various offerings as to which one might prefer. This video comparing a WE and a GHK PDW pretty much epitomises that:
What you end up buying depends to a large extent on what floats your boat, and occasionally whether you like 'tinkering with stuff. A quick surf around this and other airsoft sites will quickly confirm that the vast majority of airsofters like gadgets and toys, and like tinkering with them and customising them almost as much (and in some cases more) than they like shooting people with them. So in regard to the biggest bangs for your buck, if you are the sort who likes to mess about with tools and build or customise something yourself, then 'the biggest bang for your buck' might start out with buying something relatively inexpensive as a base platform, and then spending more on it to make it something which is vastly improved and more specialised and personal. The WE M16 A3 is one which people do that a lot with, many people turning it into a Designated Marksman Rifle (DMR). An even cheaper route for doing that, would be to take the inexpensive A&K SVD Dragunov (which when stock, is not not a gas blow back), then modify the living hell out of it to make it a semi-automatic gas blow back (there are kit parts which will enable you to do this, thus making it a self-loader, like the real SVD Dragunov DMR is, and that's not a bad thing either with the A&K, as it is quite hard to cock it by hand). The A&K can be had for as little as 130 quid new, so we are not talking megabucks here, and a plus point for two-toners, is that it actually looks okay in two tone black and green, since apart from the polymer stock - which in fact the present real SVDs also have - it is made completely from decent quality black anodised metal. The popularity of the SVD Dragunov in movies and computer games, and its somewhat macabre reputation as 'the Taliban Widowmaker' in Afghanistan, means it has a big following among airsoft modders, thus there are a lot of aftermarket upgrade parts for airsoft Dragunovs, everything from replacement wooden and polymer stocks, upgraded springs, sears, pistons etc, with even replicas of the Dragunov-specific PSO-1 telescopic sight found on the real thing. There are even online forums dedicated to modifying airsoft SVDs.
So having an airsoft rifle which can be modded and tinkered with is a bit of a hobby in itself even when you are not shooting the thing, and a good 'evening hobby for the Winter". I'm not a sniper when I do airsoft skirmishing, but even I couldn't resist getting an A&K Dragunov to mess about with, in fact, i was up far too late last night doing exactly that, modifying how I have its PSO-1 sight fitted to it and messing about with the trigger pull. So if the notion of messing about with guns as well as shooting them appeals, then think carefully about what you might like to buy, although of course you don't have to just buy one weapon.