Advice:Airport TimeCapsule,which one to connect 2.4 or 5Ghz?

If you enabled the option to assign a separate name to the 5 GHz network, then you will need to "point" your Mac at that network name if you want to connect to 5 GHz.

Using System Preferences > Network > WiFi > Advanced, you can set the connection order for your Mac, so for example, if you wanted the Mac to connect to the 5 GHz network as a first choice, you would move the name of the 5 GHz to the top of the connection order list, then Click OK, then Apply to save the new settings.

One thing that you might want to note here.......and that is that 5 GHz signals are not as strong as 2.4 GHz signals, so 5 GHz really only works best when you have your Mac in the same room, or close proximity, or close to a line-of-sight situation with the Time Capsule.

Another way of saying the same thing is that connecting to 5 GHz does not necessarily insure that your Mac has the best quality connection.

Some users swear by using the option to assign a separate name to the 5 GHz network and some do not. If you decide to use Apple's default settings, which assign the same name to both the 2.4 GHz and 5 GHz networks, your Mac will automatically connect to what it thinks is the best quality signal.....which will not necessarily be the fastest signal, since the Mac in general prefers signal strength over signal speed.

You are welcome, but I am not sure why you marked your "thanks" as the solution to your question, which may confuse other users.