Efficiency is measured by shining a light on the panel at a level of 1000 Watts per square meter, and then measuring the power output of the panel. If the power output is 140W (for a one square meter panel) then the efficiency is 14%, if the power output is 200W then it is 20%.
You may see two efficiency numbers, the cell efficiency and the panel/module efficiency. The cell efficiency is tested by just measuring the actual solar
cell output, the module efficiency includes all the dead space (space between cells, frame, etc.) on the module that is a combination of many cells and so will always be lower than the cell efficiency.
Higher efficiency only means that you can squeeze more power out of the same area. A 140W panel is a 140W panel. If one is more efficient than the other then it will be smaller. So, if you have limited space (most boats), higher efficiency will allow you to get more power out of the same amount of space. Within reason go for higher efficiency, but you will have to do your own cost/benefit analysis to figure out where "within reason" falls.
On the "same material" note, even if you only look at silicon, and set aside all the other cell types, there is a wide range of efficiencies because there are a lot of variables in design and manufacturing. Sunpower, for instance, uses "back side contacts". If you look at most panels you'll see a network of wires on the front face that collect the electrons. These, being between the sun and the silicon, reduce efficiency but are easier to manufacture. Then there are the silicon "dopants". None of the silicon is pure, the manufacturers deliberately introduce impurities that are intended to reduce the resistance to the flow of electrons. These formulas are closely guarded secrets. Some dopants are quite expensive and quite difficult, others not as much. The mix in these formulas is probably the largest difference between efficiency measurements. Then there's the glass (or other substance) on the front of the panel, what are its light transmission
characteristics (and associated costs)? Lots of variables in the mix.
Each manufacturer will tell you theirs are the best, and some modules have benefits in terms of low light performance, or construction, or... but for the most part you can just look at the rated power output to compare modules/costs. Being aware that you will never get that power from the panel. For best comparisons, see if you can find (and then use) the NOCT power ratings for the panels. NOCT = normal operating cell temperature and is a better reflection of actual panel output that the STC (standard test condition) rating that is what is advertised. You'll see some panels drop more from STC to NOCT than others, you want the higher NOCT rating, especially if you will be installing/traveling in the tropics.