An ipad isnt a replacement for a laptop. Regardless if you've been using it as such for the last few years, if you get an air (a real laptop) and use it you'll be amazed at how much better it is than the ipad.
I honestly cannot think of any reason why it would be better for you to get an ipad over the air. the air runs OSX etc. As far as portability goes, the air is crazy portable
I agree, but Karen is right. The OP has not stated that not having a laptop has impeded him in any way and with that, getting another iPad may be more beneficial. From a personal standpoint, I'd get the MacBook Air as well as keep the iPad and have the best of both worlds.