First. It is a different world. I only went first once. London to Singapore on British Airways - and on duty so it was free. Amazing. Business class is light years better than cattle, and First is same degree better than business.
First Class, and also far and away the more expensive. Example is London-New York with BA, around £1,200 Business Class, something over £3,000 First Class.
I once had to take a load of stuff to Hong Kong. It worked out cheaper to go business class as you got a bigger baggage allowance compared to economy. But upon arrival in the orient everyone was ushered through to exit the 777 via the front doors so we had to go through the first class cabin. I thought business class was cushy but first was something else. I bet it cost something else too.
As we all gaped in awe I could almost hear the head steward in a Lancastrian accent say "oh dear dear me, just come and have a look at where you could have been sat for the last 11 hours"
The layout of the seats tends to be more private and exclusive in First but funnily enough, I almost prefer Business because I like to have a few people and a bit of atmosphere around me.