I prefer the upper deck.. you'll notice one main annoying thing about UA's business class seats.. there is absolutely no storage space at your seat of any kind other than the magazine rack (which is only good for storing your menu) and a small ledge under the TV screen...where nothing can sit for takeoff and landing. However on the upper deck if you choose a window seat you have a large storage bin you can use for your laptop bag, pillows/blanket, and other personal items and when it's closed it makes for a nice place to set your drink, or a book, or whatever. The overhead bins are small and a normal sized roll aboard will not fit, but there is a closet at the rear of the cabin to store it during the flight. Just be sure to take out whatever you need before stowing it. You can get to it during the flight but it's a hassle.

I strongly recommend taking a rear facing business class seat..when you lay flat your head will be slightly above your feet which is more comfortable than the other way around..when you're in a forward facing seat. The beds are lie flat but the plane's nose is always slightly upward during flight.

The lower deck has normal sized overhead bins but no side storage bins. Even when on the window you'll probably be able to get out pretty easily without disturbing your sleeping neighbor unless you're a really big guy. You'll find UA's business class seats are very similar to CO's lie flat BF seats.. they're a little narrower though.

If your fare is booked in J, C, or D, you could transfer your CO miles over to your UA account (which is now free and instant) and upgrade yourself to first class. The first suites are really nice and worth trying out at least one way. If you're on a Z fare you can also upgrade with miles but there is a copay. Business to first upgrades are generally pretty easy to get.. if you post your flights and dates I can tell if you the upgrade can be confirmed immediately.

Here's the chart that shows the upgrade mileage cost: http://content.united.com/ual/asset/...updated_v2.pdf