The choice of whether to buy a Surface Pro or iPad 4 is based on the same factors as for most other devices. What applications does it run, will it reliably perform the required jobs, and is the cost within budget? Answers to those questions depend largely on the intended user: Will they be consuming or creating? What are their most-used applications and on which platform are they most comfortable?
Those answers are unknowable to us, of course, but questions of budget can be addressed right here. Apple starts iPad 4 pricing at $499 for Wi-Fi only models with 16 GB of memory. Microsoft's entry-level SKU costs $899 but includes 64 GB of memory (only 29 GB of which is available for user storage). For $929, just $30 more than Microsoft's entry-level Surface Pro, Apple delivers its top-end iPad with Retina display, 128-GB system memory, and Wi-Fi and cellular capabilities. So unless you absolutely, positively have to be running Windows, Apple offers the best overall value when compared with Microsoft's tablet products.