Eventually you will need top quality tools if you want to work in the competitive field of audio production. Starting out gives you the opportunity to experiment with all of the tools you will need for pristine audio production. Some of these tools are distributed for free from the producer in order to get free beta testing, feedback and make improvements to their offerings. There are certain differences between applications as some may have fewer options than others but ultimately if you need pro sound then you need pro audio tools.
Having stated that fact let’s take a look at the differences. You might find yourself looking at guitar modeling applications. The professional applications might include several makes and models of popular guitar amps while the free version might only contain one make and model. Both of these applications will do the job but the paid version will have more features, effects and scripts for automation. Sometimes the resolution will be higher in paid versions than free versions, depending on what the producer of the application was trying to achieve when they wrote the program. Do you need 1000 different effects or just one good guitar amplifier with a touch of reverberation?
Use your best judgement because both applications will be useful. While a free version might not have 1000 effects they typically have a very true and distinguishable sound. Remember the “Keep it Simple” rule unless you are planning on using some very outlandish effects. Alternatively your DAW may come with certain effects like chorus or reverberation so you can use the free amp application and then add effects before or after the amp application in the effects chain. This solves some of the paid application problems but it is usually good practice to use bundled applications that contain everything you will need for your project. This is a good way to save the settings of all components in one file and makes it simple to recall them later.
Both have advantages and both applications are useful so at the end of the day simply ask yourself what you need and how much you can spend from your budget for audio recording tools.