Shotblasting will get into the very small nooks and crannies that the wire brush won't; it will also uncover any seriously thin metal by blowing straight through it, so you'll know where the weak parts are. Wire brushing is good enough provided you don't polish the metal too shiny for paint to adhere, but in my own experience won't leave as good a finish. It's all down to your personal taste and the kind of finish you want; perhaps I was just too lazy to wirebrush the entire thing?
With reference to your springs - I wirebrushed my old Herald ones but fitted new shocks; with the top plates shotblasted and painted they came up looking very well and I got a few more years out of those. Toyota blue, I think that was - it was in a bargain clearance box...but you can see how they started off - example on the left