I opted for the 35, but only because I have an array of fast chargers already, plus my docks at home and work are 90+watts and those two places in particular are where my laptop spends most of its time on a charger (and capped at 80% using the "Al Dente" app). The 35 makes a great couch-side charger and I have both a lightning and a USB-C hanging off of it, so it mostly charges my iPad and Phone (and it does those at full speed).
Now, if it were me, and the included charger was going to be my primary, I'd have opted for the 70w, but keep in mind there are some great (if not superior) after-market chargers like the Anker Prime series that are also great buys.
Also, I wouldn't worry too much about battery degradation, but if you are, use something like Al Dente to cap at 80%.
This thing still nets me about 8-10 good working hours using only ~40% battery (80->40) (low power mode, 50% brightness, 4 tabs in Chrome, 10 tabs in Edge + Zoom, Teams) + a smattering of Apple apps like messages, calendar, maps, photos, (and the rewind.ai app which is recording the screen and meeting audio the whole time!). I don't think it makes too much of a difference but I have a 16gb ram model.