One thing to consider is what you will see when you look over your desk. Some fly tyers like a cocoon of a tying space while others, like myself, enjoy a nice view over the desk.
A desk with a tall back will provide additional storage and the intimacy of a private area. Many tyers believe that they can produce better flies when they can focus on the fly -- with no distractions from the other side of the room. And if you're desk is against a solid wall, there wouldn't be a view anyway.
A desk with no back allows it to be put infront of a window or facing the television or facing a sitting area. This also allows for flies to be tied by natural light during the day.
No recommendations here. Simply a matter of choice.

My desk came flatpacked from Walmart for $79.