Band-in-a-Box for Mac 2020 includes updated software that improves how audio is time and pitch shifted. This improvement can make audio sound better when the program makes extreme adjustment to audio so the audio will match the chord sheet. For that reason alone 2020 audio may sound better than the same audio created by 2019.

If you're purchasing either package with the intent of finding a lot of content that matches " Alternative(not grungy) with electronic type undertones" you might be disappointed. Both packages offer content in a variety of genres. Check out the demos to determine if the examples matches your music vision.

Again remember the ProPAK and MegaPAK contain the same program. So the ProPAK will not sound different than the MegaPAK. My advise was to get one or the other if your main intent at this time is to learn how to use the program.

You will have a better chance of finding content that matches that song vision with the 550 hours of MegaPAK audio content than the 340 hours of ProPAK audio content. At the same time, the audio content in either package is minuscule when compared to the 3,100 hours of audio content in the UltraPAK or Audiophile.
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Jim Fogle - 2021 BiaB (829) RB (Build 5) Ultra+ PAK
Cakewalk - Zoom MRS-8 recorder
Desktop: i7 Win 10 build 2004, 12GB ram 256GB SSD, 4 TB HDD
Laptop: i3 64bit Win 10 build 2004, 8GB ram 500GB HDD
Music at: http://fogle622.wix.com/fogle622-audio-home