Before we start I will admit I've barely used any of these sites except for GoAnimate, so don't expect in depth reviews. The best I can do is give you my initial impression, ask you to watch their video and then go and try them all for yourself. I may do proper reviews at a future date.
What makes GoAnimate stand out from the competition is that it can keep up with your growth as an animator and film maker. You can get some very professional looking results sticking with the content available but if you want to branch out and start animating your own characters and props using flash animation software you can. Import these back into GoAnimate and take advantage of how easy their studio is to use as well as still having access to their extensive library.
GoAnimate has evolved into a site promoted heavily for business explainer video creation but coming through it's initial hobby animator route it includes several character themes ideal for story telling including Anime, stick figures, Lil' Peepz, Politics and more.
For all round versatility and ease of use it can't be beaten.
Their studio is kind of like Microsoft Power Point with a cooler library of clip art and more features geared towards animated presentations. If you just want to create videos with graphics that fly in and out of each scene Powtoon is a great option that's a little less complex than GoAnimate.
Just like Wideo its content library isn't that extensive yet but it does have a few unique styles that no other site really has. For example Moovly has the most convincing animated, whiteboard marker, drawing arm I've seen yet.
You could probably do everything Video Rascal can do in GoAnimate, Powtoon, Wideo and Moovly but not anywhere near as quick. It's not that pretty just yet but with more templates and development it could be the go-to site for text based explainer animations.
Unfortunately Plotagon is a huge download (over 800MB) and I have to say I was pretty disappointed by its limitations. Again Xtranormal's desktop software, Slate, was a smaller download with greater abilities.
On the plus side the 3D is more realistic than Xtranormal and the interface is set out almost exactly like how you would format a screen play.
Currently it may be more useful as a pre-visualization tool, since the software chooses the optimum camera angles for you based on established film making principles. If you were making a live action film you could use the chosen camera angles as a guide for your live action camera placements.
I'm sure there are other sites out there - please let me know of any free sites within the comments and I may feature them in a future post.
Personally I would recommend GoAnimate to you every time but I would suggest you at least try the others, as I've become so accustomed to GoAnimate's studio that everything else tends to be quite limiting. If you're coming to these sites with no prior experience you may prefer some of the simpler studios in comparison to GoAnimate's highly evolved offering.