Growing from seed is a good way of getting lots of specimens going all at once and Trichocereus spp. typically grow pretty quickly so there's not too long of a wait before you've got a nicely sized specimen. However, you'll be dealing with unknown genetics so there'll be a process of weeding out in favour of plants with desired characteristics.
With a cutting, if it comes with assured providence regarding desired characteristics then you already know it's worth the growing space to have it. However, there's only so much of a multiplication rate for biomass with cuttings, which gets trumped by growing from seed as you can simply plant as much seed as you have space and take it from there.
The deciding factor between these two is down to budget and availability, one could say. You can get quite a lot of seed for not much money, whereas cuttings can be rather expensive - unless you happen to encounter a friendly cactus grower who might well be happy to share a known good specimen with you.
I've had success with growing cacti both singly and with multiple specimens in variously sized pots or troughs. Plastic pots are actually better, if you were wondering about plastic vs. clay. Bigger pots with more cacti are more cumbersome to move when the need (almost inevitably) arises. A nice arrangement of cacti can look very good though.
Beware, once you start a cactus hobby you'll soon be looking for every last place you can squeeze them in, I'd wager! I hope you've got a large enough house/flat
“There is a way of manipulating matter and energy so as to produce what modern scientists call 'a field of force'. The field acts on the observer and puts him in a privileged position vis-à-vis the universe. From this position he has access to the realities which are ordinarily hidden from us by time and space, matter and energy. This is what we call the Great Work."
― Jacques Bergier, quoting Fulcanelli