Definitely personal preference. I use my sling more than my carrier I think. I use my sling in the grocery store, while I'm teaching, while walking, while I'm out and its cold or windy. I also have one I use in the shower. I only use my Ergo carrier around the house right now while I'm doing chores and LO doesn't want to be put down. My reasoning is that I have trouble getting the Ergo on by myself. I'd imagine that when LO is bigger and can hold my neck while I snap the Ergo in the back that I'll use it more, but for now the sling is the easier option because its simple and one handed (and more comfortable IMO).
(FWIW, The slings I have are all TaylorMade slings. I have a French Twill and a Shower/Pool Mesh Sling. Maya Slings are awesome to. DO NOT buy a sevenslings! They're total crap and a scam!)