Honestly, I went through this whole debate myself when I was in a similar situation where I was debating if I rather purchase an iPad or the new Macbook Air. Like you, I owned an iPhone4s and a old Macbook Pro during that time. Nevertheless, at the end I came to a conclusion in purchasing an iPad over the Air.
The reason why I purchased an iPad instead of the Air was primarily because of school and the connivence in carrying it around from place to place. As a business student, many of my lectures were presented in a powerpoint slides format and with the iPad I was able to view my slides and take notes on the slides directly with my finger or a stylus. This App is similar to like paint on a Windows computer where I would directly write, circle, draw, etc. on the slides. Furthermore, I also purchased a keyboard at a pretty low cost for my iPad. I was able to type notes but navigation on the word processing at times become tricky. Another down side when it comes to an iPad is that I'm not able to write a paper on it. Using my Macbook Pro is more connivence in getting the job done.
Also, browsing works just fine with the iPad and the Gaming Apps is a bonus. I now carry my iPad to school, work, and traveling. It is a great product. On the other hand, the iPad is pretty much a blow up version of the current iPhone 5 that you current own. And if you are planning to get rid of your old Macbook or not. Yet it all depends how you use either one of the product and the purpose you need them for. I hope this helped you in making a decision.