Sounds like you want the MBA. But you could get a light windows notebook and ipad for about the same cost. Even buying an ipad 3 and later selling and buying a later ipad would be cheaper than the MBA. It depends on how much word processing you'll be doing.
I'm 48, but will be 49 in a few months so I know about the eye issue..(laughing). There is still a place in this world for laptop computers, be it Mac or Windows. I have both a MacBook '09 and the iPad 2 and I use them both for different reasons. When I want to do serious computing, I grab my MacBook, but for browsing and reading and games, I grab the iPad or my iPhone. If *I* had to choose between a MacBook Air and an iPad 3, the MacBook Air wins hands down.
The Macbook Air is going to be much more relevant in 4 years than an iPad...because the Macbook Air will be capable of upgrades to newer OS's when they arrive over the years, where as the iPad, after a couple of years of iOS updates, will start becoming obsolete (like you've seen with the iPhones since 2007).
The Macbook Air will probably deteriorate faster over that time when it comes to battery life and over all performance, but another great part about the Macbook is the ability to continually upgrade the internals with newer and better stuff (especially since technology continues to get smaller and smaller...in a few years the current Macbook Airs will probably seem bulky, lol). The downside to the Macbook is that for the initial price, you'll pretty much be able to buy two iPads, so replacing an aging iPad wouldn't hurt your wallet when comparing the two.
Then there is always the features argument...the Macbook Air can do EVERYTHING the iPad can, but that does not go the other way. For now the Macbook is still capable of having a disc drive/DVD drive added (via USB), more storage (and native ability TO store things on it via hard drive), ability to add more SSD storage space, USB connectivity PERIOD and the ability to use software like Office, Creative Suite, etc. These things may find their way to the iPad eventually, but as it sits now, they are not readily available (or the ability to use them requires a jailbreak).
The iPad to me is pretty similar in replacement time frames as the iPhone...every 2 or so years you'll need a new one (as history as showed us thus far) to stay relevant and to be able to properly use the new iOS features as they arrive.
Longevity and Features = Macbook Air
Price and Convenience = iPad
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"Speed has never killed anyone; suddenly becoming stationary...That's what gets you." - Jeremy Clarkson iPhone 4S 32GB JB'ed (finally)
iPad 2 32GB WiFi JB'ed on 5.0.1
Macbook Pro - 13" i7/1TB HD/8GB RAM/OSX 10.7 Lion
Apple TV 2 w/ 2TB External HD
I've actually been trying to decide this for myself. I've been debating on getting the refurbished 2010 MBA at the Apple Store and then waiting until my birthday this summer to see if I can con my family and in-laws to get me an iPad 3. I mainly want one for word processing, emails, surfing, and maybe some picture/video editing down the line. Cost-wise right now, the iPad is the cheaper way to go. But I think the added functionality is worth the extra few hundred.
Get the MBA. I have the iPad first Ten and have run out of space with it. I have it for sale in the marketplace, but need to get the 64 GB iPad since I like to load videos on it while traveling. Also, I am running into limitations when I open Image intensive PDF's. There just isn't enough memory to properly support large PDF's (greater than 25MB). with lots of pictures. Else, you can get a netbook so you have the traditional computing capability on hand. Otherwise, the iPad is a very enjoyable device to have.
Had an original iPad, ended up selling it, passing on iPad2 and bought a 13" MBA as soon as the last generation came out last July. Rationale:
Frequent business traveler - size matters. First time I opened it up, half the folks up front with me asked if that was the new chipset and all of them probably drooled over it. You don't know the joy of having a real computer that light, fast, portable and very durable so far.
full size keyboard - for a while I tried the BT keyboard/iPad combo as a substitute. Not a very good one. THANK YOU Apple for making this one backlight. Mandatory for the frequent flier. :-)
MS Office - not a compatible program - the actual deal and I like it better than the Windows version in some regards.
battery life - 11" version only has 5 hours rated, 13" up to 7 hours, iPad to 10. Best compromise
46 year old eyes - bigger screen wins. Even with progressive bifocals. Did I mention getting old sucks?
Overall, I love the thing. It's almost as portable for me as an iPad and I carry a mini-iPad with me everywhere anyway. YMMV.........
Thats exactly what i thought about when i bought my macbook air a week ago, good advice bro.