Microsoft basically owns the desktop market and the office productivity market. While there may not be a whole lot of growth left, it's a steady stream of revenue. Huge volume, low revenue per individual sale. Nice steady stock with low volatility and steady dividends.
Apple's revenue is essentially entirely due to 2 or 3 products. Namely the iphone and the ipad. While wildly popular, neither completely dominates it's niche market -- and competition is growing every day. Apple has high revenue per sale, but "relatively" low volume. Fantastic growth due to a few wildly popular products. Lots of rapid growth.
So, it depends on who do you think is a more viable company for teh long term. I think in the short term, Apple is a great company. Long term? I'm not so sure. MS obviously can recover from multiple major missteps (and has). I'm not sure if Apple can. To have so much of your revenue from so few products means that each iteration of those products has to be a slam dunk. Apple can't really afford any major mistakes with any of it's products.
Also, a large part of Apple's success lies in it's marketing and Jobs' cult of personality. The quality of Apple's products can be argued, but, personally, I don't think that any one Apple product is significantly better than any other product on the market -- and in terms of actual product features, Apple lags (ie cut and paste, etc on iPhone). But, Apple is "perceived" to be better by the majority of customers, and Jobs is "perceived" to be a technological visionary. Jobs has been through some major illnesses. I think if something should happen to him, Apple's stock could take a dive.
Damn. I can't get paragraphs to work right. Sorry.