All springs wear in after a period of time, from memory the loss is around 5-7% every 3 months but that is of course down to how much use they get, it's also best to amass all the parts you need and fit them at the same time, no point in taking it apart 3 times just to add a couple of bits each time
Which spring will also be determined by your site limits and what other parts you put in and I echo the above advice, buy right first time, PDI are very well regarded as about the best along with laylax but there is nothing wrong with most of the aspuk stuff