Onboards are simply a convenience! If you don't mind pulling out chargers, hooking them up and plugging them up each afternoon/night then yeah you might as well save the money.
Solar chargers aren't worth the time it'd take to look up info! About all you could "hope" to do with one is "maybe" and I mean "MAYBE" keep an already fully charged battery "kinda sorta fully charged" with a solar charger! They WONT charge a battery (unless you've got MONTHS!) and even then if they did it wouldn't be while you were using it and they obviously don't work at night. IF you had a solar panel the size of your homes roof then you MIGHT could charge a battery, but the little 1-3ft sq chargers you see sold as "marine chargers" are a joke.