A lot of people on this thread seem to be confused about the nomenclature; for me, it should be loose-ball bearings vs. cartridge bearings, using the word "sealed" for either type is misleading/confusing.

There seems to be quite a few opinions in this thread but not a whole lot of solid info, so I'll try to redress the balance.

The OP is correct that there is a strong trend recently for wheels to more and more often contain cartridge bearings. Almost all wheels that are sold as a complete units with hubs, spokes, and rims that are designed to work together have cartridge bearings. The main exception to this is Shimano - every one of their hubs that are sold separately and as part of complete wheelsets has loose-ball bearings.

Therefore, on mid to upper end bikes you pretty much always get cartridge bearings in the hubs except for the bikes that come with Shimano wheels. On cheaper bikes, there is still a lot of variability, often the hubs are unbranded, or are rebranded by the bike manufacturer, and they could have cartridge or loose-ball bearings. The lowest-end bikes (e.g., kids bikes) seem to always have loose-ball bearings.

I have read that a good quality, well-adjusted loose-ball hub will run slightly smoother, with less resistance, than any cartridge-bearing hub. I assume that this is partly because the preload can be precisely adjusted and partly because the bearings are smaller in hubs that use cartridge bearings, all else being equal (e.g., hub shell size and weight). However, that is only the case when the hubs are maintained properly. Unfortunately, many people don't know how to maintain their hubs, or even that it should be done. Therefore, a lot of loose-ball hubs are ridden improperly adjusted and/or contaminated with dirt. Cartridge bearings don't need much attention except when they need replacing, so are more often being ridden in a better state, and so in fact when taking the average of all wheels that are being ridden in their current condition, the cartridge bearing hubs are probably running smoother than the loose-ball hubs, even though the opposite would be true if they were to be all put in their ideal state.

If cared for properly, loose-ball bearings will outlast cartridge bearings by a significant margin, mainly because the surfaces of the cups and cones are more durable than in the cartridge bearings. Unfortunately, if they are not cared for properly and they are ridden for long enough to damage the cups, then the whole hub is finished and needs replacing. With cartridge bearings, no matter how bad the bearings get, if you put some new ones in then they should be as good as new since you are replacing the surfaces as well as the bearings themselves. So, for someone who doesn't want to touch their hubs, and wants to have a mechanic replace the bearings for them every few years, cartridge-bearing hubs are probably a better choice.

Servicing and replacing loose-ball bearings is pretty easy once you've learned the basics and you have a few cone wrenches. For cartridge bearings, servicing and replacing bearings can be a real problem and requires specialist tools to do it right, although you can often improvise with a blunt instrument and a hammer, but this should be avoided when possible. This is the main reason why every wheel I own has a Shimano hub, with loose-ball bearings; it is an added bonus for me to know that I'm also getting slightly reduced rolling resistance because of this, but I wouldn't recommend such hubs for the majority of people.