Each option has its advantages, and depending on your needs, you may prefer the monthly option.
If you develop a software with JUCE and don't want to be tied to a subscription, you may be better off buying a one-off license, which will give you rights to the JUCE 5 code in perpetuity. With the monthly license, you will need to continue to pay us a monthly fee to maintain your products on the market.
But as you say, the monthly license is more affordable, and if you're either testing the market, or confident that you will use JUCE in the long run, then it may be a better option to pay monthly.
We will release major updates every 14-18 months.