First, congrats on being on the verge of pulling the trigger! I just purchased a 2017 S75 with 41k miles after much agonizing but after I got in it for the first time, none of that mattered and I have the most awesome car I've ever owned!
I'd vote for the 70D because, despite a Tesla's longevity, the 40k miles is significant and I personally would value the extra couple of years of life (and perhaps fresher interior?) over the 30 miles or so extra range. Have you verified whether the 70D was upgraded by the prior owner to a 75D? That would narrow the range gap...not sure if you can still do that, or if it's even worth the $. Talking of range....I've been driving electric since 2011, when I purchased an early model Nissan Leaf. Talk about range anxiety! Honestly, I think the marketplace way overvalues range in Teslas, which works to the advantage of those of us with limited budgets buying used Teslas. My assertion is based on my location in SoCal, where the Supercharger network is, to me after my prior experience, shockingly convenient and reliable. I also have free workplace charging and installed a Tesla charger in my garage, both factors of course in my selection of the "base" battery. Your 30 mile difference is less than 30 minutes of driving at freeway speeds, so even on a long trip would the slightly more frequent stops be a big deal? Have you taken a look at the Supercharger map to see if it works for you?
Whatever you decide, an early congratulations!