This is a very old thread, but I've recently discovered both of these apps, and very quickly built prototypes for ideas that I've had.
They both have their strengths.
First things first, costs: both are FREE. In every sense of the word. Blynk has a pay model in place, and it seems very reasonable. But, if you install the Blynk server, then connect your app to that, you get a very large number of "credits", and if you login to the web-based admin interface for the server, you'll find the credits are entirely editable for you. In that way, with a modicum of extra effort, you can use Blynk completely without cost. I used the docker container and it was up and running in a few minutes.
Also, the Blynk server and device code is entirely open source, more on that later.
Blynk is entirely focused on IOT, or device-based (Arduino, Raspberry PI, ESP, etc.) development. What this means is that the app can add buttons, graphs, gauges, text boxes, etc., however there is no ability to edit the programming logic within the app, you have to write the programming logic/code and put the code in place on your destination device.
This is great in the sense that if you're used to, or focused on, IOT development, then you get to add a nice app-interface on top of your IOT project very quickly.
Where this may fall short, is generalized app development. You won't be using Blynk to make a catalogue of your plans, or a photo gallery, but you can use Blynk to monitor the moisture level of the sensor you have installed in your plant.
MIT App Inventor development:
No cost, entirely free mobile app development, with a high level of confidence given that it's been around for a long time and being backed by MIT, the author's of the MIT open-source license.
With App Inventor, rather than an IOT-focused app development environment, you get a general purpose app development environment. It's based on Google's blockly programming language, so all programming is done by dragging various blocks (they look like puzzle pieces) around.
AppInv has built-in support for database storage, and a straight forward layout system. It may lack a little bit in the asthetics, but blockly coding is a lot of fun.
There are extensions out there, I think I've seen a demo that allows voice-based programming, and I imagine the sky's the real limit there.
There's a variety of ideas I've had, and it's been like "sheesh, writing all that code will take forever...", but AppInv is opening those doors and should allow me to bang out a few of these ideas quickly (my first app, a road game we play as a family, took me 30 minutes or so).
And then, the convergence. AppInv has the ability to post to web-based API endpoints, and Blynk's open source server provides web-based API endpoints. Thus, with a still small level of effort, you could get the amazing versatility of AppInv coupled with the ease of Blynk's app-based IOT communication ability.
Whew! The sky's the limit!