dual channel doesn't mean u cant use different memory, it just means if they arn't matched, u dont get dual channel operation for a small performance boost.
whether 4gb dual channel, or 6gb single channel is better would depend on your memory usage.
if you rarely/never use more than 4gb, then running 4gb in dual channel would be the best option.
however, if you tend to go over 4gb of usage, then 6gb would give u the better boost