I have a carrier. I used it loads for dd1 as she was one of those babies you couldn't put down, so used to have her in it loads, for loading the dishwasher/ hoovering etc 
then my last 2 only had 15 months between them, and I couldn't fit a double pushchair in the car, so used to carry dd2 in it and push ds3 in the buggy.
Mine turns to front facing at about 4 months. It can pull on your shoulders when they get heavier, and make sure you get one with a back support strap! it helps to spread the weight.
I might get a sling this time.