Well it is only my opinion and I'm sure that there are many others.
If you can afford it (especially if the purchase is a tax deductible expense) then I'd go for the highest spec you can.
Personally I've not gone for an SSD larger than 512gb because I can manage with that capacity for six to twelve months and at the point where I start to feel a touch constrained I'll get external storage. All going for the 1tb SSD would do is buy me a further six to twelve months before I still have to make the jump to external storage.
I'm also not convinced regards Fusion as I believe it's a halfway step. all of the negatives of SSD's in terms of low storage capacities combined with the negatives of HDDs in terms of reliability. The only benefits are that it is faster than a HDD and cheaper than an SSD. That doesn't outweigh the negatives for me.
The teardown of the latest iMac also shows the HDD part of the fusion is Seagate. A name that many have some strong opinions about.
I was wondering how you were doing 4K video editing yet only using 100gb on your current iMac but if that is a future intention then it explains that.
As I understand it video encoding is heavily processor dependent so you should be able to get a good feel from looking at a processor comparison between what you currently have and the 6700 i7.
That said there appears to be little performance gain between the processors in the 2014 models and the new ones. I'm not sure what it is between the 2013 and 2014 models.
Geekbench is showing a score of 4354 on a 64bit single core for the 2014 i7 model compared to 3913 for the 2013 i7 model.
I would hazard a guess and say that the 2015 i7 will weigh in around 4800 if it is a 10% improvement on the previous CPU (all other things being equal) which is an increase of around 20%.
Hopefully we will get some real world benchmarks soon enough.