To me there's 2 ways to look at this -
First, does the 420 buy you enough over the 280 to justify the expense and are their drawbacks? This you have to answer by considering how you use the boat and if you're looking to use the boat differently (longer trips, more nights aboard, etc). I went through the same thing when deciding about going from 32'-38' The 32 got me into different types of boating then I did before, longer trips, overnight offshore fishing trips, etc, but I was getting the bug to do it in more comfort. I made the move and 10 years later don't regret it at all. I spend a lot more nights on the boat, usually do longer trips, etc. But you should also consider if those evening dining runs would be practical where you are going (do your dining spots have big enough slips for the 420?) and if you'd fish as much on the bigger boat depending on how far you run, fuel costs, etc. Will an increase in draft impact you where you boat? Depending on how you use the boat going up can have its drawbacks. Think about how you use the boat, how you want to use the boat, and if the bigger size makes sense.
The other option you seem to be considering is not having a boat at all and chartering to cruise, fish, etc. As others have mentioned it's different owning the boat vs chartering. The other thing to consider is availability of charters, where you are looking to go, how much planning is involved, etc. One of the great things about owning a boat is you have an easily accessible vacation spot which needs minimal planning depending on what you are doing. A few years back friends of mine who were never boaters bought a boat because they got tired of dealing with weekend trips, cancelled flights, etc, and just wanted something where on the spur of the moment they could get away. Sometimes I wonder if I didn't have a boat what else would I do? You mention 55 days tops - when you think about it if you have 52 weekends off a year and let's say 4 weeks vacation, that's 125 non-working days a year. If you're on the boat 55 of them now that's currently 45% of your days off on the water at some point. Replacing a good chunk of that with charters isn't going to be cheap either and as others have said there's a big difference in the experience of running the boat yourself (whether fishing or cruising) and going along for the ride. Then again going to different places and chartering in new territory can be a great experience also which certainly has its pluses (I did a 10 day Virgin Islands charter this spring). But if you're a long time boater I think losing the option of just going down to the boat every summer weekend and relaxing on the water would be hard to replace. Unless you keep the 280 and put the money you would have spent towards 420 on charters in different spots for a couple weeks anyway and get a sample of both.