Unless you find a closed loop for a low low price (mine was a recon unit for £40), you're almost into custom loops at that price anyway. If you plan to overclock stuff in the future, definitely custom.
In theory, the all-in-one unit. You may be one of the very few who encounters a leak from one, but they are supposed to be professionally sealed at manufacture.Which has less chance of leaking? I prefer custom for the nice aesthetics, but it's an awful lot of tubings and fittings and I don't think I'd cope very well setting up a loop.
Mine was on special offer. It would have been at least £100 back then... and for the resulting performance, I knew a custom loop wasn't that much more at the time.£40 would barely buy him a CPU block for a custom loop. If you can afford it go for custom but if not go for a all in one or a decent air cooler.
A properly put together custom loop
Completely disagree with AIO having less chance of leaking.
A properly put together custom loop is far mroe reliable due to the pump. Pump failures on AIO units and Custom loops are the main cause of leaks/bursts after the loop is working together and past the leak test phase. This is due to pump failures more likely on the AIO as the pumps are on the block (meaning they operate at higher temperatures) and they are also considerably weaker. When the pump fails, you will soon find the very hot near boiling liquid at the block trying to find its way out.
If you spring a leak on your GFX card or CPU and fry it then you buy a replacement, most warranty are voided as soon as you remove the stock cooler anyway.
I use EC6 coolant in my setup, its apparently non conductive, but im not willing to test that statement out, im sure it is in small quantities, but a massive leak or a popped off pipe, who knows !
Which is another reason why my cooling setup up is built the way it is, I use 10/8mm tubing with 3/8 barbs, you need to dip the pipe in a cup of boiling water to soften it up enough to even get it over the barbs, once its cooled back down the only way I can get it off is with a very sharp knife, I use cable ties as an extra precaution to secure it, but to be honest, I don't think they are really required, that pipe is going no-where.

