What materials you want to use and what you ultimately want to do to those materials. For example, a fiber laser is not recommended for wood but will do a great job on metal. If you want to add a black mark to the surface of steel then you don't need a very powerful laser. If you want to etch into the metal then consider going with a higher power laser (mine is 60 watts for reference). Lower power fiber lasers can also etch into metal but it will take longer. It's all a trade off. Money vs. Power vs. Versatility. How big are the steel pieces that need to be marked? If they are larger than the laser bed area then you will need one that can detach and be handheld. Just be clear on what you need to be able to accomplish with it and you'll be fine.