It more of a preference thing, you’ll find that it’s something that eventually, you’ll do both. 

Do you prefer hearing sounds in movies ‘behind’ you, or do you watch a lot of movies and enjoy the explosions? That preference will lead you to which you want to do first. Fortunately, Sonos is a flexible system, and you can do one, and then do the other when you want to / can afford to, without penalty. 

The answer is to connect the Beam to both, ideally.  I would personally chose to start with the Sub-Mini and add the surrounds afterwards, as I tend to listen to more music and general stereo TV channels, but if I watched lots of streaming movies/shows, then I would opt to get the HT surround speakers first, but look to get both eventually.

It is now possible to get a Home Theatre completion kit from Sonos. See this link: 

https://www.sonos.com/en-gb/shop/home-theater-completion-sub-mini-one-sl-pair

Edit: There is also a Klarna 3-payments option, with no fees. So that’s worth thinking about, perhaps?