I am thinking of changing to a harness for my westie pup for several reasons. He has a soft nylon collar and lead at the moment but the collar slips around to his front and the he trips over a loose lead. I cannot tighten the collar or it would be too tight.He doesnt pull ahead at the moment but can stop suddenly behind me and isnt all that good at walking yet so I thought a harness may take the pressure of the neck area and allow a loose lead without him falling over it or getting tangled in it. I think some harnesses also have a adaption so can be used as a car harness too. My westie doesnt like cars at the moment so it would help if he had a harness which could double up if poss as a car harness so I am not struggling to get him into it just for a car jouney. Please could other westie owners advise me what you use?

I have always used a plain nylon or leather collar with an extendable lead. I know alot of people dont like extendable leads but I think as long as you use them responsibly near main roads you cant go wrong. For the car there are plenty of things you can buy, including tunnels which lay on the back seat, and car harnesses. 

I don't like a harness on a westie unless they are a lot older and trained to walk and stop properly.

I think they need lead training as part of the puppy training. The harness I think isn't as effective during this time.

If the collar fits properly it shouldn't be a problem. Perhaps buy a thicker one as in the band width they tend to stay in place better.