Hi Athenaa,
Have you got some photos you could upload to the site? Would be good to see what you're working with.
From a functional perspective, other people on this site might be better equipped to answer your question.
However, from an aesthetic perspective, I would suggest considering the colours and finishes of the house and any fences that are nearby to the retaining wall before deciding what material to use.
You'll get the best effect if the retaining wall 'works with' your house/fences but does not match. That is; the colours and materials tie in, in some way to (perhaps) the trims etc. Creating a cohesive effect is key when doing any renovations or additions to any part of your property.
What you don't want to do is have the same finish on the house as on the retaining wall though. I usually like to try and pick out one of the trim colours (say, the guttering or the fascia or the window frames etc.) and use that colour on the fences/retaining wall.
Also, I'm a fan of using darker colours for fences and retaining walls because they tend to provide a more solid 'frame' around the property.
Does all that make sense?? Good luck!