Steam Link is designed to be a small remote "workstation" box that allows you to remotely connect to your PC (or any PC in your house that's running Steam). With a good network connection (ideally wired on a gigabit ethernet connection or on the 5.0 Ghz band of a 802.11ac WiFi network), it will operate practically as if you're on that computer itself (this is coming from personal experience. using an older WiFi network or a 100 Mbps router may provide a less than ideal experience, but may still be acceptable in some cases).
The Steam Link is good if you cannot physically connect your own PC to your TV (either due to distance, space requirements or room / entertainment center aesthetics) or don't want to deal with fighting with the potential wire jungle behind your TV / entertainment center (this can be a pain the deal to connect & disconnect the HDMI cable... as well as finding a power outlet for your computer) & you want to play your PC games on it. Being just a small box (about the size of a Roku box), it can typically be hidden away near your TV or in your entertainment center with little effort.
Steam Link can utilize a keyboard & mouse as well as XBox 360 / One controllers, but is more ideal with use the Steam Controller for the minimalist console-like experience, even for games designed for mouse & keyboard... but your mileage will vary with the Steam Controller (hence why you can still use an XBox 360 / One controller a keyboard / mouse). If you wish to use other USB-connected devices (like a USB steering wheel or other special peripheral), you can purchase "Virtual Here" ($12) to extend support to the Steam Link.
I will note that the Steam Link will tie up your computer while in use, as the games will be physically running on your computer & Steam will be pushing the video output to the Steam Link while the Steam Link pushes the controller / keyboard / mouse inputs back to your computer... so you can't have one person on the computer while you're playing a game via the Steam Link (unless somebody wants to troll the Steam Link user).
While the choice is ultimately up to you, I hope this provides some information to assist with your decision.