The shoulders of the Pikkolo are padded with Memory foam (like in those fancy pillows), so pretty comfy. I don't think it's as bulky as the Ergo, the body is canvas, with padding here and there.
I've heard people really like the Ergo for back carries - something to consider is if your kiddo's ever a 'leaner', the body of the Pikkolo is higher, whereas leaners can be a pain in the butt in a shorter bodied carrier...

Are you in the Baby Carriers Downunder google group? Google them (surprisingly enough) if you're not, they've got a sort of Carrier Library - because what I'd really recommend is that you try both before you buy. I bought the Pikkolo without trying, and totally don't regret it, I'd heard enough good reviews to go for it - but if you've got a tricky shoulder, then it might be worth seeing which you prefer before investing.