I started with Esprit 5, then ummed and ahhed about whether to upgrade to Infinite, and I haven't regretted it FOR A MINUTE. 

I couldn't do without a lot of the advanced features now e.g. the ability to tweak rendering parameters more, the Open Ocean function, the Ecosystem painting and so on.

But at the end of the day, the question is "What do you want to do?" - your answer to that will determine what you need to buy.