Advice on selecting a lens?

I've been looking into purchasing a portrait lens and was hoping I could get some input based on knowledge/experience. My portraits consist of infants, maternity, family, and children/toddler. I mostly take pictures of babies and toddlers. I've been reading some reviews on portrait lenses and I keep finding great reviews on the Nikon 50mm f/1.4G and the Sigma 17-50mm f/2.8. I'm leaning more towards the Sigma 17-50 f/2.8 but not sure. I own a Nikon D60 and my lenses are 18-55mm 3.5-5.6g (this one came with the camera and is what I'm currently using), and have a Nikon 55-300mm 4.5-5.6g. When I think about purchasing the lens, polarizer, and filter; I think I should start off with the 50mm. I'm thinking about selling my 55-300 mm 4.5-5.6g lens since I never use it to save/purchase the Sigma 17-50 f/2.8 in the future. I really just want to make the right choice for my first portrait lens and any advice would be greatly appreciated.

I have Nikon's 17-55 f/2.8 and it is a useful lens on my cropped sensor body. The Sigma would be also, but read reviews to see if the quality will be acceptable.

The 50mm f/1.4 is an outstanding portrait lens for DX bodies, such as your D60.It gives you more control of the background blur and costs 1/3 what the Sigma costs. The 50mm f1.8 is supposed to be almost as good for only $150.

I would recommend the 50mm f1.4G over the 17-50mm f2.8. Why? For portrait purposes on a crop sensor camera the 50mm is equivalent to 75mm, very close to the 85mm lens preferred by many pros on full frame cameras (I use the 50mm when I don't have space, the 85mm indoors, the 105 mm outdoors or in halls where I have space- the longer the focal length the more flattering it generally is to the subject.).

The focal lengths below 50mm on the Sigma really are not very useful for portrait purposes. For portrait purposes the Nikon 50mms very large aperture, which at f4 is exceptionally sharp and will have a great bokeh (background blur) is pretty much a perfect portrait lens on your D60 (the 85mm becomes a bit large except for when you have lots of space since it is 130mm equivalent n your camera)

Would I recommend the 50mm f1.8 over the f1.4? Only if cost is really an issue. Don't forget that crop factor applies to aperture as well- so you are really talking f2.7 vs f2.1 (and f4.2 on the Sigma). The optics and build on the f1.4 are also superior to those on the f1.8, but you need to decide if the difference is worth the extra price (me, I use the older 50mm f1.4D so no guesses as to which I would recommend lol)