I plan on buying a new MacBook Air, 35W or 70W charging adapter?

No, not at all.

Why not get both? Specify the 70W option with the Mac, and buy the 30W adapter separately, for travel.

30W USB‑C Power Adapter - Apple

Or if you want to charge the MBA plus an iPhone or something else at the same time,

35W Dual USB-C Port Compact Power Adapter - Apple

Cost is the same either way but in my opinion the 70W version is more versatile, and you can use almost any USB-C power adapter for iPhones / Watches / etc.

It really, really does not matter. You can leave them plugged in all the time. Their respective battery charging algorithms and options are nearly identical, and are designed to maximize battery longevity whether we want to or not.

Don't leave either one to discharge and remain in that fully discharged state for a long time (months or years). That would be bad. They will die like a neglected Tamagotchi.

"I really wish I didn't get so much memory" — said no one, ever.

Zachyy wrote:

One more thing, I know that for iPhones it’s not particularly good to leave it plugged in fully charged,

You can absolutely leave iPhones plugged in when fully charged. Even before Optimmized Battery Charging, iPhones would stop accepting a charge when they reached 100% and drain a little. Now, there's optimized battery charging. Use that and don't worry about it.

It's up to you. The 70W adapter can be used to "fast charge" that MacBook Air but it is larger and heavier.

Fast charge your MacBook Air or MacBook Pro - Apple Support

👍

It's a no-brainer then. Few decisions are as easy.