You didn't mention how tall you are. Manufactures have different measurements with their frames. For example, I am a L or XL depending how the manufacture designs the frame.
To answer the question in the title, a frame being the correct size is the most important thing when selecting a bike. If you are between sizes, finding the correct size is something you need to figure out. Take both bikes on a 20-30 minute test ride (longer if possible) and pick which feels best.