The normal pros and cons come into play.
March will offer longer hours, more entertainment choices, probably less refurbs BUT it likely be busier because of Spring Break crowds and probably more expensive. Also, as this last week has proven, unpredictable weather.
May will have less crowds, and lower cost BUT you have shorter park hours, less entertainment choices and a few more refurbs are likely. May weather is usually wonderful in So Cal.
So much depends on how you want to honeymoon. If you are interested in gung ho touring of Disney; see and do it all, then March is probably the time to go. If you want a more laid back vacation, plan to enjoy the hotel amenities and dining, then May is probably the better choice.
Myself, I would choose May. Enjoy the later gate opening times while lingering in bed with your new hubby, take advantage of the earlier closing times and plan late meals at the resort hotels. Maybe even throw a trip to the spa in there. You could still do all of that in March too, but missing rope drop will make it harder to collect fast passes, and you might feel you are missing prime entertainment options if you leave the parks too early. KWIM?
BTW, Congrats and best wishes! Been happily married for nearly 17 years.
