Metal frames are modern looking. They go with photos, children's art work and needlework, humorous pieces and stylish stuff. Wood frames are for Victorian pieces, outdoor scenes, religious pieces and serous stuff. The metal frames should match or compliment the mats which compliment the piece. Wood frames should be as light or as dark as the mats are and be fairly close to the type of wood in the room they will hang in.
Frames also come in many widths.
The fancier frames are best used on complex, intricate work not photos and modern work. If you have problems with mat selection and this is all taking just a little too long, walk away from your work and look at something else for a few minutes. How to Determine Frame and Mat Sizes Measure the design at its' widest and longest points. The frame size is the same as the mat set. Many needlework pieces don't readily fit common frame sizes but the is a way to sometimes make it work anyway.