Which look better for a frame 1x2s or 1x4s?  

Hi pc

I would stick with the 1x4 due to the increased stiffness. Stiffness is proportional to the width (height) cubed (to the third power). The 4" will be 8 times stiffer than the 2". Furthermore, the 1x4 material is actually only 3-1/2" wide, so ripping it in half will yield less than 1-3/4".

More specific to your question- I like the looks of a wider frame. If you cover the frame in black velvet or some other very black fabric/paint, it should look very nice.