mev

1. One of these choices is a self-supported ride, the other three are supported tours. Those are a bit different, so I'd probably also decided depending on the type of the trip.

2. I use "weatherspark.com" to look at climate averages. It tells me the likelyhood of rain each day in March is:
~30% in Charleston SC
~25% in Daytona Beach FL
~10% in Tuscon, AZ
~30% --> ~20% in Monterrey, CA
So if you are deciding solely on rain chances, then southern Arizona has the driest climate that time of year.

Another thing to recognize is your east coast locations have a higher average annual rainfall; ~50 inches for NYC, Charleston, Daytona Beach - in contrast to Tuscon ~12 inches and Monterrey ~20 inches... so you are picking part of the rainiest period on the CA Coast, but that is also relative to a lot more arid parts than further east.

Also note that this part of the coast is already drier than further north, e.g. Astoria, Oregon has annual rainfall of 80 inches and over 50% probability all of the days in March.

3. Despite the feedback you received, I personally would still consider a California coast trip. I've done that stretch two different times during the wetter time of year: one in December over Christmas break and once leaving San Jose on March 1st and heading southbound. On both occasions I got some rain - at times even a fair amount (a big storm hit Monterrey just before I left on the Christmas trip, but was drier after; there was five inches and flooding in Lompoc on my March trip)...however, it also seemed that the weather forecasters could often see the storms several days out and if one has some flexibility with days, one might be able to dodge a completely bad day.

However, if the main objective is to dodge rain and a supported trip is preferred, then Arizona looks like it meets your criteria better.