Generally Soarin' has always had the longest wait times. I am not sure if FP+ has changed that at all but I would choose that to use a FP+ on personally.
Neither are scary in a traditional sense. Although Soarin' can be if you are scared of heights and can give you butterflies in the stomach sensation. Maelstrom is dark boat ride with a drop not sure if your children deal with the dark well?
If your children are tall enough to ride, then I would FP+ Soarin. Personally, I love Maelstrom, but my kids much prefer Soarin. Also, the wait times for Soarin tend to be pretty long for most of the day, whereas you can typically find decent wait times for Maelstrom.