I switch back and forth depending on my mood, the amount of time I have, and the fabric I'm using. In general I think a rotary cutter is faster to use and can be especially accurate if you have experience with it. But for thick or slippery fabrics I think a great pair of dressmaking shears is nicer to use. I generally weight the patterns if I'm using a rotary cutter and pin if I'm using shears. When I just want to relax and enjoy the process (I'm weird about cutting and prepping since, unlike many people, I actual like doing it) I use shears.