1. The big differences are in the manufacturing process and initially mono was considerably more efficient and expensive. But technology has advanced and efficiency is nearly identical now. Mass production has also leveled the cost differences. There are some very minor advantages mono has in high temperature efficiency and low light performance, but not enough to bother worrying about.
2. All good larger panels have internal bypass diodes per group of cells, which means you lose only the power output from the shaded group (not even the entire panel), but the current flows through the bypass diode, so rest of the system stays productive.
To choose your panels, first determine which size you can fit, as that's usually the most important factor on a boat. Note that the "standard" panel size in most large solar farms is 1x1.64m - if you can fit this size, it'll give you by far the largest choice of panels and the best prices. If not, pick the largest ones you can fit, at least two panels for MPPT series wiring (higher voltage = better), then drop their stats in an Excel sheet and calculate a few things from them:
Cost per watt (€/W): Simple. There are some companies specializing in marine solar panels and want to sell you one for 3x the price. Don't get ripped off. Many are just rebadged panels, others have somewhat more corrosion resistant frames. But the standard house roof panels are absolutely fine on boats (they have to pass salt-spray corrosion testing, as houses in coastal areas can see a lot of that), and usually more efficient as they have to keep up with the market instead of focusing on a niche.
Power per area (W/m²): Manufacturer efficiency specs are often poor and confusing, as some specify cell efficiency and others panel efficiency. This simple method gives you a better comparative value and includes space wasted on the frame.
Power per weight (W/kg): This is an interesting one. Panels and frames are not constructed equally, and weight matters on boats. Too heavy a panel is obviously bad, but too light can also mean the panel glass is too thin and can shatter easily, or the frame is too flimsy and not rigid enough. So avoid very heavy and very light panels.
Happy shopping! When I fitted mine, I used 2x LG LG300N1C-G4 - but that was early 2017 and the technology moves incredibly quickly, plus the prices shift rapidly too (mine cost £519.90, incl. delivery, which can be costly), so you'll have to do your own research. By the way, you can adjust prices - the same panel was also available in 290/295/305/310/315 watts at different prices - this is due to selection during the manufacturing process. The extra 15W cost a lot more!
