Welcome, Newcomer! First, let me address your primary concern. Dual channel was designed to address the bottlenecking that occurs at the memory controller. This bottlenecking happens when the system memory (RAM) is unable to keep up with the processor.
The dual-channel configuration alleviates the problem by doubling the amount of available memory bandwidth. Instead of a single memory channel, a second parallel channel is added. With two channels working simultaneously, the bottleneck is reduced.
So, in short, dual channel is the better option.
Now, as for your concern of having more density (GB)... Most real world applications don't have the use for more than 4 GB of RAM. If you are doing any kind of rendering type of work, you will see the need for more density. Although rendering type of work is memory-intensive, it still supports and benefits from dual channel.
As I had mentioned above, the dual channel technology was designed to reduce the bottlenecking. So, even if you did opt for the 1x2 GB + 1x4 GB SC route, you may still find that your system would be struggling. Remember, in dual channel mode, you have twice the bandwidth, which equates to double the processing power, more or less.
One last thing...
Mixing RAM is ill advised. There is much more to discuss on this subject, so I won't bore you with the details. Just know that when it comes to RAM, it is always best to use identical sticks. And not just from the same company or same timings, etc. I'm talking identical as in the same lot. The best way to ensure that happens is to use a kit.