Microsoft Certifications

Most important to note about both of these certifications is that you're expected to already have proficiency in the areas of study they represent before you attempt to certify. They document your knowledge of vastly different technologies, so depending on what you already have knowledge of, you'll want to select accordingly when constructing your certification plan.
An MCITP (Microsoft Certified Information Technology Professional) focuses on any of several Microsoft server systems technologies, and documents knowledge of those products and their application in a business IT environment. The entry-level MCITP focuses on Windows Server and Active Directory (and related, commonly installed tools,) but there are several MCITP options (and growing regularly) that include Exchange Server, SQL Server, and Sharepoint, to name just a few. The difficulty of these exams varies widely, and your actual knowledge of the products will also play a role in how difficult you perceive these exams to be. In general, the more experience you have with the actual product the easier they will seem.
CCNA (Cisco Certified Network Associate) focuses on networking concepts like OSI model, subnet math, Ethernet switching, routing, and a basic introduction to routing protocols and would be appropriate for a network admin with a couple years of experience who wanted to demonstrate proficiency in those areas. Relatively recently, many new CCNA options have been added, with the 'classic' CCNA having been renamed to 'CCNA: Routing & Switching'. There are also security and voice CCNA options, which, obviously, focus on network security and IP telephony. Which CCNA (or CCNAs, if you elect to get several) you choose to acquire should be based on what you know and what you're interested in. Like the Microsoft exams, any of the CCNAs require experience with the product, and in this case, I'd recommend extensive experience with the products, and an encyclopedic knowledge of subnet math before attempting any of them. All of the Cisco exams assume the student has the ability to do subnets (I.e. binary math) in their heads or with limited note-space, and will ask questions that you can't possibly answer correctly (without guessing) if you don't know how to calculate subnets in relatively short order.
Whichever you choose to pursue, make sure you have access to an adequate lab, have the official curriculum for the exam, and a good study guide to work through.