The Artisan 2000 or 1600 calico

I have spent a lot of time looking at your advice page but am a little confused. I am interested in either the Artisan Pocket 2000 or the Artisan 1600 Calico but unsure regarding firmness and link with spring thickness. My partner and I would both like a fairly firm mattress but like others the difference in weight is considerable ie 17 and 11 stone. Would medium suffice if I bought the Artisan 2000 but with a variation in spring tension to accommodate the difference in weight? Would the 2000 not provide more support than the 1600 or does it depend on the springs and do the springs determine firmness? One of us sleeps predominantly on the back and the other front/side, does this have an impact on what I should buy? What would you advise?