The Deathadder is only £33 new.
The MX518 can be had for £20 or so used. The MX518 has been replaced by the G400, but dunno how expensive the G400 is.
The Deathadder has flawless tracking. The MX518 has mouse prediction. If you don't know what prediction is, it's when the mouse corrects your pointer when drawing straight lines. It makes drawing straight lines much much easier in paint, but you lose some control because of it. Some prefer prediction, most don't notice it, some hate it. It's mostly preference.
Gaming mice are a luxury. They make your gaming (and non-gaming too) experience better by having smoother tracking and generally a nicer feel. Once you use a high quality mouse like the ones I mentioned above, using a cheap one again will feel pretty terrible. Whether or not the higher quality sensors make a difference is entirely up to you. Like I said, I can't use mice with positive acceleration. I use low sensitivity so I swipe my mouse very quickly over pretty long distances. Positive acceleration is quite noticeable for me.
edit: the
Logitech G400 is only £25 in the OcUK shop. Amazing mouse for the price. It's pretty much the same as the MX518 except Logitech listened to their customers and fixed the prediction. So it has flawless tracking like the Deathadder.