I'd highly recommend the G&G, especially if you're not bothered about it being metal. The sports lines are made of Nylon Fibre and they feel great, not that far removed in feel from the metal ones. You can pick them up for around £130 as well.
Additionally, if you're worried about the blowback being a bit crap and gimmicy - not travelling the full distance, jamming in the middle and putting unnecessary stress on the internals, then don't worry about it. G&G use a pneumatic system, the blowback is powered by residual air, the bolt travels the full distance (on the MP5s anyway) and since it's air powered, it doesn't have any direct connection to the gears or piston whatsoever so it doesn't add wear to the gearbox and physically can't get jammed in the middle.
On top of that, it gives the gun a really great sound and it actually looks pretty good, as well as it also making the charging handle fully functional, which is awesome fun to lock back and slap down. If it weren't for the fact that the cylinder head portion of the gearbox was visible through the ejection port, people could mistake it as a GBB - Though there is no felt recoil.
I'm firmly in the camp that blowback is a bit... gay, if it's merely for the aesthetics, but G&G have done a good job on the MP5s and it does actually add to the gun, I enjoy it.
There's also the fantastic G&G hop rubbers in there, my MP5 can hop .40s to the same level it can a .20. It can probably hop even heavier than that, I've just never bought ammo heavier than that to test it. With .25s the performance is faultless, it comes shooting at around 320 - 330 fps on .20s, range is a good 40m or more and the accuracy is amazing for a stock gun. I've not seen anything perform better with no upgrades apart from a G&P M4. It's so good that it's the only gun I've ever had that I've not even fitted it with a tight bore and I have the Version1 Top Tech version of the MP5A4, so it doesn't come with a TB as standard like the V2s do - before anyone says "that's because it comes with a tb anyway."
I honestly can't recommend it enough. First AEG I ever bought. Best AEG I've ever bought. I've never even opened the gearbox. Still got it, still love it. It just doesn't see much use anymore because of the L85 and me playing almost exclusively in woodland.
The ONLY thing I dislike about it, is that it's incompatible with most MP5 external parts and for some reason, the rof with a 7.4v Lipo is stupidly sluggish, despite it being a good 700-800rpm with an 8.4v nimh.
Ideally I'd like to front wire mine into a tac light handguard and fit a sliding, collapsible stock. Maybe also get the A3 receiver for a proper SAS embassy look. But the receiver is the only thing I've found that I could actually swap because the G&G tac light handguard has horrible RIS rails on it and the stock is the kind that still sort of sticks out a bit even when it's fully collapsed. They're also both £80 which is just stupid.
Also, in response to WhiteDeath's comment above, ICS sportsline MP5s can be bought new for £100. Though I forget where from, I imagine you've been on the site already if you say everywhere is out of stock.
Though I do primarily play there I want something for when I visit friends etc!
