Which lens would you choose?

- a Nikkor 50mm f/1.8
- a Sigma 70-300mm F4-5, 6 DG Macro

It's for my Nikon D3100

Added (1). Yes, I know they are two different types of lenses, but I still have trouble deciding. Right now, I only have the kit lens. I'd especially like to do some portraits.

Do you want 'selective focus' shots where the subject is sharp, but the background blurred? If so you need the wide aperture of the 50mm f1.8, or even the f1.4.

Do you want to photograph things that are far away, wildlife, sports etc? If so you need the reach of the 70 - 300mm, but you sacrifice aperture to get it.

Only you can answer that. In time you will probably get both.

Since they are all useful for various types of jobs, I'd have to know a lot more about your style and planned usage to answer.

I have a canon and I use the sigma lens you mention. I also have the 50mm lens, it came with my camera but I use the sigma one more as it is more versatile and gives you more options. I also have a macro lens but that is meant for close ups and portraits more then the other two.

I use my sigma zoom lens for long range objects as most of the things I take pictures of fall outside the 50mm distance.

Basically the answer would depend on what kind of photography you do.

50mm normal every day wide angle shots
70-300 mm distance shots
Macro close ups and portraits.

You can take macro shots with all of these lenses if you change your camera settings.

Do some research on what the benefits of each lens are.

Until you know what you would use a 50mm for vs a 70-300mm lens, you might potentially be wasting your money on a lens you may not need or want.

For portraits… Definitely go for the nifty fifty… 50mm on an APS-c sensor like on the D3100 will provide slight perspective compression and is useful for producing shallow depth of field.

The Nikor is a prime, shallow depth of field. Good for portraits, flowers etc.
The Sigma is a tele, has a good reach. Good for wildlife, sports etc.
(edit) If you want to do portrait, definitely get the 50mm. Best budget lens on the market.

Actually, both should be in your camera bag. It's like comparing sports cars to off-road trucks: both gets you there, but one can't beat the other at what it does best.

Get both, eventually.

EDIT: The 50mm, then. Shallow depth of field for creamy, out of focus backgrounds.