I would say Shift Key, because you can use both hands to reach it so for example if the letter is on the left you can reach out to the Shift Key on the right. Yet I'm not an expert

Shift, definitely

Shift. Hitting Caps Lock requires you to hit the key twice to toggle it back to lowercase after you've keyed the capital letter. Shift only requires you to hit they key once.

Software developer here. For me it depends. Almost always, I find shift to be faster and more comfortable. However, sometimes I have to type things like THIS_LONG_ALL_CAPS_VARIABLE, in which case I find toggling capslock before and after to be faster and more comfortable.

Absolutely the shift key -- it requires only a single press which can be performed in tandem with striking the letter to be capitalized, whereas the caps lock key requires you press it, strike the key you intend to capitalize, then press it once more to turn it off -- this requires literally triple the effort of using the shift key.

This isn't even a real comparison. If you're playing to type quickly, always use the Shift key.
Unless you're chickenpecking keys like a geriatric, CapsLock should be used exclusively for very long strings of upper-case words.

Thanks for the all replies people. Truth is I've been using caps ever since I've started typing. It's clear to me that shift is the right one to go with. Time for a transition!

Make the caps lock do something more useful!!!!! Have another backspace or control!!

UNLESS YOU'RE TYPING LIKE THIS, It is going to be better to use the shift keys.

Well, caps can be useful. If you do happen to need MORE THAN A SINGLE LETTER IN CAPS FOR SOME REASON, it's a life saver. Really just trying typing even two or three words in all caps using the shift key and you'll see why caps still has a place on the keyboard.