The iPad, even with a keyboard, will not do "just as good as a mac book". They are different tools for different purposes.
The question boils down to "what do you wish to accomplish with this device?". The laptop (any laptop) is far superior to any tablet at this point for creating content - from school papers, to spreadsheets, to work applications, to media (video rendering, photo editing, podcasting, etc). Sure, there are iPad apps for some of these tasks, but the true computer (MacBook, iMac, Mac Mini, etc) are far better suited to these tasks. Plus they have substantially more storage capacity.
The iPad, on the other hand, is great for consuming media - reading books, surfing the web, watching video, etc.
Even with a keyboard, switching between tasks such as surfing the web/doing research and writing a school paper will become incredible frustrating... plus the lack of a USB port or media slot makes managing files (slightly) more difficult than on a computer.
So, why do you want the device? What are you going to use it for?