Well if you go in April, depending on your dates and locations you visit, you could enjoy the spring trees blooming (not just sakura). If you go in October, depending on your dates and locations you visit, you could enjoy high season of fall foliage.
Fantasy Springs in DisneySea should definitely be open by October. It may or may not be open when you go in April. That may make April have lighter than usual crowds, with people waiting to visit until Fantasy Springs is for sure open. Of course, an opening date may be announced before you finalize your trip and that could make April have different characteristics that I'm writing above. I guess for me, I'd likely choose October, just to let some of the novelty of Fantasy Springs wear off. Hard decision. Best of luck deciding!