For me, Southwest is ALWAYS cheaper than Delta, Jet Blue, or American, but it does definitely depend on where you're flying out of. For instance, I live in CT and flying out of our small airport is super cheap for SWA nonstop, but flying out of NYC or Boston is never nonstop on SWA and is WAY more expensive.
The 2 free bags per person is huge especially when going to a place like Disney, and they are not free on other airlines. With a fee of ~$25 per bag on other airlines, that can really add up if you have a lot of them. Not having assigned seats is not an issue for us, but like PP poster said YMMV. The more people you have the more annoying/stressful it can be, but if you check in exactly 24 hours before and get a good boarding position you will be fine.
SWA flights generally do not show up on Expedia, Orbitz etc at all. SWA releases their next batch of flights on May 19th for travel through January 4th. Once those have been released they will update their website with the next release date, which will most likely include your April 2017 flights. I have never gotten a cheaper price on SWA than I have when booking the second the flights are released (around 6am EST the day of the release), though other people have said their prices drop sometimes. If you do book a flight and the prices drops, you can get a credit to your account for the difference.