Monitor specs for a MacBook Pro 2015

• connection (Thunderbolt 2 or HDMI)

the preferred connection method is [full size] or [Mini] DisplayPort. This protocol uses a packet interface and puts the smallest load on your computer, because it is not a "heartbeat refresh" protocol.

(DisplayPort family is roughly equivalent to the more expensive ThunderBolt and the less-common USB-C)

Your Mac contains separate display generators for each possible display you can attach. But it uses shared display memory, so the more pixels you show on all displays, the harder that display RAM must work. 15-in and 16-in (and 17-in) MacBook Pro models have a private display RAM used for all displays when external displays are attached. Most others try to share system RAM.

Using a "heartbeat refresh" display (including the built-in display) puts stringent timing requirement on the rate at which display data muct be supplied, and is more taxing.

When you choose [√] Mirror displays, the Mac uses a compromise resolution that both displays should be able to handle, and uses ONE display generator for both displays.

You have to decide what monitor works best for you then check that it works with your Mac. We have no idea what resolution you want, what refresh rate, what size, etc. best meets your needs. We could list a hundred or more compatible monitors but that would not help you in the least. You need to tie down your monitor requirements.