Sorry in advance, I'm not going to tell you which is easier.
They're very different subjects to learn. I got a 43 in Enviro and a 44 in further last year.
Further is way more straightforward - you know what you need to learn and you have plenty of practice questions that will be near identical to the exam.
Enviro is significantly more complicated, the study design isn't super clear and the only resources that exist are all made by the same company. The textbook has missing information - information that I struggled to find on the internet (whilst also having a hard time figuring out what it was I was supposed to know). There's very few company practice exams (same company that makes the textbook), there's one lecture during the year (same company) and that's it for resources.
Further can be a risk though because making a silly mistake on the exam can significantly affect your score, whilst in enviro the consequences aren't quite so severe.
My experience with enviro was probably heavily affected by my teacher though - we had many, many issues with him throughout the year which we complained about, but none of us were believed and nothing was done about it until after the last sac
tl;dr further is straightforward but can be risky, enviro suffers because of lack of resources but you're less likely to damage your score because of a few silly mistakes on the exam.