People who bad mouth the 28-300 or older 35-350 seldom speak from the experience gained by owning one. Both are best in class & I own both, and rely on them heavily. The 28-300 however is rather heavy & that can be an issue but after several years of using either of these lenses I have a hard time knowing whether many of my photos were shot with the superzoom or my now sold 100-400 L IS (version 1). The IQ is just too close to call between the 3 lenses. I've taken the time to add some samples & put the shot info on them but unfortunately they won't show all that large here so I'll add a link to one of the forums that has larger images to view.
And from the non IS 35-350 L which was obsoleted by the introduction of the 28-300 L IS.
And this is the link to an owners / users group which features sample photos including some of mine starting on this page (message 1620 & a few pages later). I had other samples there earlier but the albums were removed (by me) from the hosting site so no longer exist
https://photography-on-the.net/forum/showthread.php?t=256450&page=108
You will get a lot of advice, all of it sincerely and well meant about what lens is best to use. However you must consider that advice against your own requirements, which I think boil down to some very specific questions:
How much do you want to invest?
Whatever your budget is, stick to it.
What is your level of interest ? E.g. casual, student, enthusiast, semi-pro, professional.
The more to the right you select the higher your investment and commitment.
What types of photography do you engage in? Another way to say this is what types of INPUT will you engage with?
Your answer indicates a general interest, so will a single focal length range suit your needs?
What kind of OUTPUT do you intend?... by which I mean what do you want to produce? This is a critical question as it identifies the investment you need to make to produce that quality. Are you looking at prints for competition, for large format prints, smaller prints (say A3 and below), to produce images for digital display or to put on web pages? This is a very broad range and the demands vary accordingly.
What are you prepared to Carry?
The best camera is the one you have with you. The 28-300 is a bit of a howitzer! If you are prepared to deal with the weight it's an amazing piece of kit. I don't mind carrying the camera and body for a decent length of time, but I do so using a holster with a good wide support strap for my own comfort. I also use it with a battery grip, both for the extra capacity and the balance it gives to the camera, but that's just me.
I have a lot of lenses, many of them L-series of different ranges, among which is the 28-300mm lens and I have found it extremely useful when I don't want to carry multiple lenses or change lenses in the field - something I am loath to do.
I would recommend looking at Youtube.com for reviews of this lens and also sites like DPreview.com and the Digital Image. By and large the reviews are very favourable, but if you can do so I also recommend renting one for a short period of time to see if it will work for you. It's a big investment and worth the research.
I have used the lens on a wide range of crop and full-frame bodies and have found it works well on all. The 70D is a great camera still. I apologise if you know this but the Angle of View will alter depending on whether you have a crop sensor or a full-frame one. The numbers on the lens for focal length are correct for the lens, and PHYSICALLY it will not alter, however the Angle of View (i.e. what the lense "sees") will vary depending on the body it is used on. So on a FF body what the lens will see will be the 28-300 as written on the lens, but on a crop body like the 70D, the Angle of View is altered by the factor of the crop. This perception is known as Equivalence, thus the 28-300 will seem to behave like a 45-480mm lens.
(to explain Equivalence further I recommend the DP review article:
https://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care
or the excellent Youtube video https://www.youtube.com/watch?v=lte9pa3RtUk ).
To demonstrate how it can work on the humblest of bodies, here is an image (reduced in resolution for this site) taken hand-held with the 28-300 on the venerable Canon EOS 400D (a 12 YO camera) - I often take my old gear with me for a casual shoot. Personally I am happy with the results, especially since I was not aiming for professional perfection, but I did want to capture a moment when the shadows fell in a specific way to separate the statue from its background.
Details: 97mm, f/8, 1/500, ISO 800
