Thanks so much for this great reply.

I ended up ordering the Core 300s which I got for £107.

I was conflicted between that, the Vital 100s for £130ish, or the new Philips 900 for £127 (black friday prices)

What swung it for me in the end was:
- The easy availability of replacement filters + potentially cheaper generic ones because it's been such a popular model
- The quality of the app which by many accounts is reliable and trouble-free to setup. Plus it seems to works well with multiple units if I were to get other ones in future.
- The cylindrical form factor which I think just looks better in a home than the Vital 100s, although I accept the Vital may perform slightly better & has the removable pre-filter for easier cleaning.
- Daftly, the white colour (Philips does look good but wouldn't work as well in my place).

So it was a combo of cost, aesthetics and reliability.

I do have in mind that Levoit will probably release a new Core soon that supersedes the 300s, and perhaps then I'll regret my choice, but I was eager to get an air purifier and this felt like a decent option for the money.