Which shears are best for dog grooming: straight or curved or both?

It depends on the coat you are grooming. I use curved shears for shaping curly coats on Poodles, Bichons, and to trace the foot of my Beardie. Straight shears are useful for everything else.

However, I get the most use out of my thinning shears. I use them for EVERYTHING. You'll probably want a good pair of curved, straight and thinning shears.