In another thread several comments were made about the inferiority of the 75-300 zoom lens. (I am a beginner but I have one of these and I understand the issues). If memory serves me correctly the 70-300 was the recommendation.There are two versions at considerably different price point. I can't specify a budget because I'm sure my CFO would say I don't have one.
Hi and I was the one who posted the review of the 70-300 series HERE .
Either of these lenses would be better than the 75-300 units, so really it's down to what you can get budget for. Canon had some of the MkI version as a deal recently - I have located the lenses in question at that site HERE , although the MkI version is out of stock at present.
As always much depends on what you are shooting, and producing. However, if your comment about a 'CFO' is to be taken literally, as opposed to your domestic partner, you are looking at a business where the quality of your images is a part of your value, service or product presentation, then it is worth getting glass the will do your work justice.
What exactly are you shooting and what is your intended output: i.e. large prints, digital display, web and social media? All of these have different demands.
Well we have in common that we are retired and shoot wildlife. I am on my own now so I can spend what I like (within my own limits), but I got this gear when my other half was alive, so parallel situations. If you sell your 75-300 it would help to ease the financial burden and shows good will to the 'SO'. What camera are you shooting with?
The lens should work well with that, and as Kvbarkely wisely emphasized, you will gain the benefit of the excellent image stabilization of both lenses. The later unit is faster to find focus, but both are good units, so the rest depends on what you can get in your location. It might be worth going on the web and buying from a reputable source like B&H, who I think ship internationally. Another source for used gear is KEH - they are pretty well respected as far as selling used gear and clearly identify the condition of what they sell - they ship internationally. HERE is a link to their website with the Canon 70-300 lenses isolated (along with some other brands. They will also trade or buy your old lens and if you sign up to their newsletter you get 5% off.
If you are on a budget, then the best telephoto zoom lens for you is the Canon EF-S 55-250mm IS STM. It is a very sharp lens that some people say is as good as either version of the 70-300. You can probably find a good used one for about $150.
I agree with Mike that it is a good lens, and certainly a better optic than the 75-300, but it doesn't have the reach of the 70-300, and for wildlife (as identified as the activity by the OP) that is a critical factor.