Nikon D3100 Lense advice please
 

I currently have this lense for my nikon d3100. I have this one and the standard lense.

http://www.jessops.com/online.store/...6332/show.html

The above tamron lense is a great starter lense and it does everything i want , i can get some lovely shots with it but now i think its time i want a lense which can zoom that bit further and maybe slightly crisper shots when full zoomed as this one can get a little grainy at times.

I dont want to spend a fortune and i dont really know much about lenses so im asking for your help! Lense does need to have auto focus btw

I walked into jessops yesterday and noticed a lense which was 70-300mm like my tamron , yet it was £269 and the tamron like half that price. So whats the difference? All a bit confusing for me being a bit of a beginner!:confused: