But still which one among those two has better heat spreaders?
The X Series modules look
better to me
(which is why I bought them for my own rig), but performance wise, there's no difference in how they cool.
Second part to you question, it's just Single Channel vs. Dual Channel (1 module vs, 2). In general, Dual Channel is better, but it's not a huge difference, it just adds more available bandwidth.
With that said, I'd go with the 2x4GB kit myself.