I have the ergo backpack, it's useful for front carries and hanging from the pram for a long trip, but I found it a bit tricky to use in a back carry as I didn't like having bag and bub on the same side - Alex is boof enough as it is without adding weight. Has heaps of handy pockets and stashy spots though, so hanging onto it, might see how it goes with a wrap one of these days.

As for getting a sling as well - what sort of sling would you be looking at? Are we talking ring sling, woven wrap, other?