I have a Canon 600D with a 70-200 f/4 L lens, and I often photograph people outside in bright daylight. Now, I always shoot RAW and do some post-editing in Photoshop/Lightroom, but I'm not sure how to get the best most lifelike result? With under or overexposed? This, of course, is if I can't get it correct the first time!
Overexposed areas have a sharp cut-off when the maximum pixel value is reached. You have a completely white area if this happens in all colour channels. There is nothing you can do in post to recover information in those areas.
In contrast, underexposed areas retain information, but when you brighten them, you amplify the noise, too, and with less bits to record details, you loose gradation.
So, in conclusion, a light underexposure allows to still process the image to an acceptable result with some noise reduction, while blown out whites are lost for good.
Still, the best course of action is to expose as bright as one can get away with without blowing out any important highlights, a technique called "Expose To The Right (ETTR)".
Neither. Underexposure means that you did not deliver enough light and you are losing information in shadows. Overexposure means that you delivered too much light and losing information in highlights. Both are wrong. Expose correctly. Check your histogram or allow the blinking under/over exposure alarm in the preview to see how are you doing. Correct if necessary.
Shooting people in bright light is fine, but if you combine dark shadows and very bright areas (like sunny day at noon), you will have problems with tonal range of your pictures. Try to avoid extremely high contrast situations for normal shooting. Find better location, better time of a day or use some of the techniques that equalize the contrast (reflector, flash, better selected angle to the sun...)
When the composition requires detail from shadows I would suggest exposing to the right on the histogram making sure not to overexpose and blow the bright areas on the photo as that would mean losing information in the shot.
Taking photos in raw with some post processing skills you will be able to get details from the shadows. But harsh bright sunlight will always screw with the contrasts so consider exposure bracketing and/or dual-iso (magic lantern - warning might brick your camera and void warranty).
I am not sure how to get the best most lifelike result?
If by "more lifelike" you mean you want to avoid dark shadows and squinting subjects and instead capture smoother gradations, softer shadows, and more natural facial expressions, then stop shooting in bright sunlight and instead move your subjects to areas with open shade.
Hard light means light from a small or distant source like the sun; it's very directional and casts a strong shadow. Soft light means light from a large source, like a large area of open sky; soft light comes from many directions, so the shadows are much softer and more pleasing, and it's also much more comfortable.
Areas shaded by large objects like buildings provide plenty of soft light without the problems caused by very bright, very hard direct sunlight. Open shade means areas where the sky overhead isn't blocked, but the sun doesn't shine directly on your subject.
Shooting on cloudy/overcast days is similar to shooting in open shade: clouds block the direct sun and instead give you large light sources with soft shadows.
When taking the picture you should use the expose to the right (ETTR) method as explained in ths' answer. When doing the post processing work you can then decide to reduce the exposure to let the scene look more naturally. E.g. if you take a picture at night, you can usually crank up the exposure to make it look like daytime. This may be what you want, but perhaps you want to show how the scene actually looked like to you which will then be a lot darker than what the picture looks like. As explained in the other answers, it's then still advantageous to expose for as long as possible and then reduce the exposure in post processing as that will reduce the noise relative to taking the picture with the exposure you want straight away.