Saying things a bit differently, if you're going to put a valve on to restrict flow, then there's no reason to use a bigger pipe as your primary return. If you turn the valve closed, you're turning a 1/2in drain to maybe 1/4in (or smaller), at the valve. The flow volumes are most affected by the tightest constriction in the pipe. From a basic theory standpoint, imagine instead of turning a valve you are installing a narrower and narrower drain pipe until flow is perfect.
Now if you're going to be "installing" a virtually narrower pipe, then there's no reason to use the bigger drain as the main return, since it doesn't help that process. Again in theory it shouldn't matter which you use, but in practice there's a bit less things that can go wrong with the bigger pipe as your backup. Imagine by having a trickle going into the backup, it starts getting algae and snails living in it. The bigger that pipe, the less the odds of the backup getting plugged.
Does any of that matter? If you're not at the max limits of your piping, it's likely not going to matter. If it were me, I'd focus on making sure that even if your pipes are closed, your return section is small enough that it won't overflow the display. If you do that, it's impossible to overflow, even if the pipes completely plug.
Regarding valves, just buy a cheap ball valve and install a union. You'll get it tuned and then barely mess with it. Then you'll mess with it and get a hang on adjusting it. Then maybe you'll get annoyed with the lack of precision, and you can later spend crazy money on a gate valve. You don't have to spend that money now if you don't want, as long as you're ok swapping it later.
Bestest setup: big pipe as backup, gate valve, unions
Other setups will all work too. You can even later swap which is the primary versus backup. Just turn off the pumps and move pipes around. Use unions ahead of time to make it easier