5x7s and 6x8s are so close they're almost the same thing. In fact, a lot speakers have "5x7/6x8" as the size because they can be used in either application. In general, bigger speakers give better bass. In order to produce good bass, a speaker needs to be able to move a large amount of air at once. The larger the speaker cone, the more air can be moved. That being said, 6x8s will not necessarily give better bass than 5x7s because like I said before, they're practically the same. In that case it just comes down to the quality of the speaker.
I know you said you don't want to fill your truck up with subwoofers, which I understand and agree with completely, but there are small subwoofers that don't use much room at all, and some of them can even fit under your driver seat, using up virtually no space at all. I think it would be worth considering if you're looking for more bass.