A monogram consists of the three initials, and places the last initial in the middle and larger than the other 2 initials. Initials are simply three initials, straight across, first-middle-last, with all the characters the same size.
Which style you choose and which style is best is generally a matter of personal taste. I am able to engrave initials either way based on your instruction. Please get in touch if you have questions about initial engraving.
What is the difference between a monogram and initials?