I would put my money on a stock JG over Aftermath any day. I've always insisted and stick by my guns (no pun intended) that the Kraken is a good project gun for someone that wants to eventually upgrade it to full metal and wood without having to fork out all the cash at once. Over time, you can have a gun that will shoot better and be far more solid than a stock plastic TM, and will cost about the same. Stock however, I wouldn't trust a Kraken as far as I could throw it. You'll need a bunch of internal upgrades out of the box to keep it going.
If you're talking stock, JG is the way to go. I've had JG, I play with guys that have JG. Most of them have run stock JG M4s for 3-ish years now and haven't had a single problem with them without even openening them for preventative maintenance. I would put my money on JG any day. Out of my entire collection of 17 guns, I'd have to say the single toughest most problem-free gun was my JG MP5. I've never seen a gun take that kind of abuse and keep on working as great as the day I got it.