As listed in the specifications for the EOS Rebel XSi/450D included on page 181 of the EOS Rebel XSi/450D Instruction Manual, the 450D is an E-TTL II camera.

Both the Yongnuo YN600EX-RT II and the YN685 are E-TTL II flashes. Either will work in E-TTL II flash metering mode. More recent Canon camera models, as well as some older higher end models, include additional capabilities and control options of E-TTL flashes directly from the camera's menu. But either flash will allow the full capability of your camera.

Both flashes have the same power and zoom coverage. The only real differences are in their wireless off-camera capabilities.

• The YN685 has a built-in radio receiver that uses the Yongnuo YN622 radio protocol for remote E-TTL triggering and control (or can receive the Yongnuo YN560/RF605/RF603 in manual only triggering and control mode). A YN622 compatible transmitter (such as the YN622C-TX flash controller or YN-622C II transceiver) must be attached to the camera's hot shoe for E-TTL, HSS (High Speed Sync), Multi, and 2nd curtain sync (A YN560/RF605/RF603 controller can only control manual flash power and flash head zoom). The YN685 has no optical slave or optical wireless capability.
• The YN600EX-RT II has a built-in radio receiver ses the Canon RT radio protocol for remote radio triggering. A Canon RT compatible controller or flash (Such as any Canon r Yongnuo flash with 'RT' in the model name, or a Canon ST-E3-RT or Yongnuo YN-E3-RT) must be attached to the camera's hot shoe. The YN600EX-RT II also has optical slave modes (any bright flash of light can trigger, but not control it) and optical wireless capability using the older Canon optical wireless protocol (a compatible Canon or third party 'EX' series 'Master' flash or ST-E2 optical controller on the camera can both control and trigger it).

As listed in the specifications for the EOS Rebel XSi/450D included on page 181 of the EOS Rebel XSi/450D Instruction Manual, the 450D is an E-TTL II camera.

Both the Yongnuo YN600EX-RT II and the YN685 are E-TTL II flashes. Either will work in E-TTL II flash metering mode with your EOS Reel XSi/450D. 

More recent Canon camera models, as well as some older higher end models, include additional capabilities and control options of E-TTL flashes directly from the camera's menu. But either flash will allow the full capability of your camera.

Both flashes have the same power and zoom coverage. The only real differences are in their wireless off-camera capabilities.

• The YN685 has a built-in radio receiver that uses the Yongnuo YN622 radio protocol for remote E-TTL triggering and control (or can receive the Yongnuo YN560/RF605/RF603 in manual only triggering and control mode). A YN622 compatible transmitter (such as the YN622C-TX flash controller or YN-622C II transceiver) must be attached to the camera's hot shoe for E-TTL, HSS (High Speed Sync), Multi, and 2nd curtain sync (A YN560/RF605/RF603 controller can only control manual flash power and flash head zoom). The YN685 has no optical slave or optical wireless capability.
• The YN600EX-RT II has a built-in radio receiver that uses the Canon RT radio protocol for remote radio triggering. A Canon RT compatible controller or flash (Such as any Canon or Yongnuo flash with 'RT' in the model name, or a Canon ST-E3-RT or Yongnuo YN-E3-RT) must be attached to the camera's hot shoe. The YN600EX-RT II also has optical slave modes (any bright flash of light can trigger, but not control it) and optical wireless capability using the older Canon optical wireless protocol (a compatible Canon or third party 'EX' series 'Master' flash or ST-E2 optical controller on the camera can both control and trigger it).