I use all three depending on the time of year and what I'm doing. I generally use the trunk bag to pack lunch and a few other small things on the way to work, in cooler or wet weather I usually like to have more room for cloths so I use a pair of panniers. If the weather is fair and I'm riding the mountain bike over the hill, I take a backpack. The backpack I have has a hard contoured back and when I ride I get some decent airflow and I'm not as sweaty as with a regular backpack. I wouldn't want to use the backpack on the road bike, I feel like it wants to slop around a little more, where as on the mtb I'm more upright and it hangs where it needs to.