Auto mode or manual speedlight?

Hi i'm thinking of buying a speedlight. What do you suggest.a ttl or a manual one?

Also when is it used?

I have a nikon d3100

Modern dSLR cameras are fully automatic. Using a manual flash, negates those advancements

Stick to dedicated flash units designed for your camera. This means something like a SB400 or if you can find one, a good used Nikon SB600

A flash unit is one area where you will find even the pros are ok with the automated TTL mode. It does quite well and will save you lots of time and headaches using a fully manual flash. A dedicated flash in TTL mode is nothing to be concerned about using, … In most cases. Yes, there are times when you want full control of flash power due to some unusual lighting situation or simply when you are after a very specific look in the photo that automation is not going to provide. The thing is, it is very easy to use the TTL flash in full manual mode, but if you have only a manual flash, you can't use TTL to make life easier on you when you need it.

I suggest you get the best Nikon dedicated TTL flash you can afford.

TTL (auto flash metering) is ok for some circumstances, but it often fails to get it right (like most things auto) & if you don't know how to light manually, you won't know WHY its gone wrong or how to put it right.

Better to learn to light manually than rely on TTL.

You have a few choices;
- get a Nikon speedlight (expensive) - SB700 or above will do TTL and manual settings
- buy a cheaper TTL/manual speedlight (third party)
- buy a full manual speedlight (third party)

You use flash when the ambient light is;
- not enough
- the scene is contrasty & you want to use fill flash to reduce the contrast
- you want to put a 'catchlight' in the eyes

The Strobist blog is the best online resource for learning to light;
http://strobist.blogspot.co.uk/2006/02/welcome-to-strobist.html

start with the Lighting 101 & 102 archives (drop down boxes, lower right)

and off camera;
- when you want to be in total control of the light on your subject - it's direction and quality (hard/soft).

Do you know how to adjust a manual flash to get the right lighting? If not, get TTL. There are cheap third-party brands that offer TTL mode for the Nikon dSLR. If you do love working in manual mode like me, then you can get even cheaper manual flash guns.