I recently got the 891, and I had been debating between the 710d and the 891 before making a decision. Ultimately, I chose the 891 because it's an older model with limited upgrade options, and there wouldn't be any new software or features released for it. Even if I upgraded to a color screen, the difference wouldn't be significant. It's also small and portable, making it easy to carry around, and it's more affordable.
I was also considering getting the 705 to complete my collection, but I heard that new models of the 705 have been released. So, I decided to buy the 6100 instead. It's too tiring to keep chasing after the latest models; it's better to wait and buy a classic older model.