Angela, get yourself a copy of Nigel Calder's "Boat Owner's Mechanical and Electrical Manual", right now. You will love it. It will explain everything you want to know about this subject (batteries, inverters, chargers, alternators etc. etc.) in plain English and in detail. If you had it, you would not be asking these questions here.
Personally, I am out on the idea of using GC batteries as a house bank; haven't you already had some explode on you? One of the secrets to Sky's success is that he uses the big bad Rolls 8HHGs for his battery bank. They will crank the engines and provide 275 amp hours (AH) at 32 volts. That's like 365 AH at 24 or 730 at 12 volts.
I went to a separate 24 volt bank for my inverter system because we planned to, and did, live on moorings full time in winter (Marathon) and summer (Westport, MA). I just didn't feel comfortable running one of the main banks down all the time, and I had new, much lower AH (about 190) Interstates on the boat already via the PO. I also liked Magnum's integrate-able inverter, automatic generator start and battery monitor system (the new ones are even way slicker than mine). Additionally I had plenty of room to install 4 L16 deep cycle batteries delivering 410 AH at 24 volts in the generator room. They have the same footprint as a GC but are taller.
My primary goal was to have a system that would power our giant household Sub Zero side-by-side reefer, along with two beverage coolers, overnight, along with the rest of the upper deck. We ended up running the genny about 6-8 hours a day anyway for cooking, dish and clothes washing so that worked out fine. Now that we have settled in North Carolina for a couple of years, the generator is used much more for heating and cooling.
The flaw in my system that I have not corrected yet is that the inverter bank does not get charged off an alternator, so it gets run down more while underway. Someday I may hang a 24volt alternator off one of the engines, which is doable though somewhat expensive. I only have selected circuits on the inverter and someday may add more. Given where the different AC panels are, that will be a bit of a PITA.
While I have been delighted with my system for the last 3 1/2 years, if I had to do it all over again, I would probably install a second, smaller and quiet modern "night generator" of about 10 KW (my boat is wired for one) and MAYBE hang an Outback or NewMar on the 32v house bank, which is now made up of the same Rolls batteries Sky has. Current long term plan is to get the 2nd generator which may well mean giving up my beloved Magnum system. I would run that way awhile before deciding if an inverter was needed.
Remember, inverter power is not all "free". You are going to buy the energy to recharge that bank from somewhere: your engines, the genset, or FPL. Eventually, the batteries have to be replaced. To me, the main benefit of the inverter is the quiet, and secondarily not putting out exhaust fumes and water all night.
I'm thinking all this through trying to understand it as it applies to our needs and the ideas are still all over the place and often contradictory. One thing I don't know how to do is figure out how much battery power we'll need each day to run what we want to run to even know whether our golfcart battery house bank will be sufficient or not. I don't know what each item on this boat draws or what will be on the inverter in order to do the math. So, I was sort of thinking that more is better. 
