Much of this answer depends on whether you are a racer or a cruiser. On cruising boats the boom is often used for a variety of functions. They are sometimes used as a cargo boom to haul people and gear and sometimes dinghies aboard. In that situation the topping lift needs to be sturdy. Having it rigged permanently means that you can reef on the fly and not worry about dropping the boom into the cockpit. In that case the topping lift should be chosen to be low chafe to the sails.
On a race boat the topping lift is really pertty temporary and efforts are made to reduce weight aloft so you see a lot of things. Often the main halyard is used as the topping lift on smaller race boats. Even when there is a topping lift on a race boat it is unhooked from the end of the boom and run to the base of the mast and tensioned in the the lee of the mast to reduce the windage of the lift.
There are a lot of ways to rig a topping lift. On a cruising boat, my favorite way to rig a topping lift is to rig it as a spare main halyard. This has the disadvantage of a lot of extra weight aloft so alternatively it can be rigged to serve as a messenger line for a spare main halyard.
You also see a set up where there is a fixed wire rope to the end of the boom or to a block and tackle at the end of the boom that is lead forward to be adjusted.
To some extent a rigid boom vang can do some of the functions of a topping lift such as preventing a boom from dropping completely when the sail is dropped. But in general it is not a great idea to leave the boom suspended for long periods of time on the rigid vang. In race boats with rigid vangs the main halyard is often used to hold up the end of the boom.