On the question of 'airgun rated' it is true that the action of a springer or rammer is more damaging than from a centrefire rifle but I would still claim on the warranty if anything went wrong. It never has with me using any quality scope even on the Eliminator which is the most scope-unfriendly rifle I own. On that particular rifle I use the Simmons Pro Air 6-18x40 but a lifetime warranty is a lifetime warranty and will be honoured providing you can produce the documentation that originally came with the scope. The problem of course is that the Simmons Pro Air and WTC ranges have been discontinued so lord knows what scope they would give you if you did claim.
As far as the 1.5-5x20 is concerned it is a fantastic little scope but sadly doesn't have enough mag for me so I use the Simmons WTC 3.5-10x40- which is quite a bit smaller and lighter than its big brother the 3.5-10x50-when compact size and weight is required. As I said in a previous post, I reckon the absolute best of the current generation of compact scopes is probably the Bushnell Elite 2-7x32 mildot. Lots out there to choose from so there should be something to suit us all!