Ignoring the specific lenses here, the question you’re really asking is “which is better, a general purpose ‘superzoom’ lens or a dedicated telephoto lens?” The answer to that is of course “they’re different”:
- The superzoom lens (18-200 in your case) has the advantage that it has a very wide focal length range; it can do everything from landscape photos to some wildlife photos (see below for more on that). However, nothing in this world comes for free – the image quality won’t be as good as dedicated lenses, and you pay a premium for flexibility.
- The dedicated telephoto lens will probably have better image quality for the range that it covers, and is cheaper.
Only you (or your son, or whoever’s paying) can decide which of those tradeoffs is right for you. Three notes though:
- You lose a lot of the value of an interchangeable lens camera if you only ever use one lens. Unless you’re in a sandstorm or under a waterfall, changing lenses isn’t a risky process so long as you’re moderately careful.
- While we in general don’t recommend specific products, I’m prepared to break that rule for the 75-300. Never, ever buy this lens – it’s image quality is awful and it doesn’t have image stabilisation. For a crop sensor camera like the 80D, you’re much better off getting the 55-250 and cropping the image.
- Even a 300mm probably isn’t going to let your son get good images of wildlife unless the wildlife is pretty tame; for bird photos in particular, you’ll be looking at 500mm+, but those lenses are much more expensive. Just setting expectations here.