When in April would you be going? Easter is the 12th of April in 2020, which means there are the potential for spring break schedule and crowds and Easter activities in the first couple of weeks of April. Spring break schedule usually means longer operating hours, more entertainment (especially in the evenings), and fewer scheduled refurbs. If you wait until after Easter, the crowds might be lighter, but the hours will probably be shorter and there will be less entertainment. Also, if the schedule remains the same, they will also be offering the Food & Wine Festival in DCA in the first part of April (it ended on Apr 23 this year).
HalloweenTime is one of my favorite "seasons" at
Disneyland. It is very popular, so there will be crowds, especially on the weekends (Friday night through Monday). HalloweenTime usually starts a week or two after Labor Day, so if you do plan to go in September, I'd aim for the middle to latter part of the month. The hard-ticket Halloween parties usually start towards the middle/end of September, so keep that in mind when planning your trip if you want to check the party out.
I think either time is a lot of fun at DLR. It depends on whether you want to see HalloweenTime and when in April you would be going. If it's a choice of HalloweenTime or late April, I'd vote for HalloweenTime, even with the likely larger crowds. If it's a choice between HalloweenTime and early April, I'd be happy to go at either time.