Liam G. answered 09/14/20
Experienced Musician Teaching Piano and Music Theory
So they do two different things which I think is very important to go through. The Akai MIDI controller HAS NO onboard sounds. It is meant to control VSTs like Omnisphere, but it produces no sound of its own. All keyboards that are specified as a MIDI controller actually produce no sound. So the real question is are you looking for a synthesizer to produce sound, or a controller to control a VST?
If you're looking for a VST, check out some free alternatives first to see if they fill the hole in your production workflow. Check out Helm, or the TAL Noisemaker for some free synth VSTs.
If you're looking for a synthesizer and you want a physical one, the $500 may be a bit tight but there are still some in that price range that are definitely worth looking into. The Korg Minilogue for example can be found for $500, same with the Behringer Deepmind 6 if you look for a used one. Both of those produce sounds of their own.
As far as a MIDI controller, depending on what you need the Arturia Beatstep is another classic choice, but again, it produces no sound of it's own and is meant for controlling VSTs.
Hope that helps, feel free to ask any questions!