As I get older, more fally-offy and less repairable, I’ve gradually got around to wearing knee/elbow pads and lumpy shorts etc…

Next on the list is something for a bit of spine protection.

So, your thoughts please on choosing a backpack with spine protector or an armoured base layer. I think I would rather have the base layer unless you can sway me otherwise.

I use both depending on what I’m riding.
If its short DH runs where I don’t need a backpack then I have a Race Face vest with a back protector built in which I use as a layer, for all day rides where layers come on and and off depending on the weather conditions then an Evoc backpack.
For budgets purposes, if you had to choose either, then get the vest as you can then still use your existing backpack.