Yeah, the 633. Sorry, been putting in a lot of hours.
If you want to discover where the price difference is, start comparing the actual specs and capabilities.
Each device fills a specific niche in the world of field audio. Instead of comparing prices, first delineate your needs. What are you looking for in your field recording kit? Is it personal/hobby or professional? What will you be recording? What applicable audio gear do you already have? Is "modular" the way you should go? And a thousand other questions that only you can answer.
My personal answer was to rent what I need when I need it. That was a professional/business/financial decision. The possible scenarios I could possibly face made for an expensive field recording kit. As I would not be in the field all that often it could not be financially justified.
All that said, the prices have gotten to the point where something like the DR-70d or R-44 could be financially justified. I would still need to rent for some situations, but under $1k could amortize out in two years.