Inclined axis: more load on rear bearings; less on front bearings. Rear bearing also has to take a thrust load as well as a radial load. Probably has an easier time coming up to spin speed with small loads, because the load tends to accumulate toward the rear. Front seal less critical especially with low water levels. Less vertical pounding on front bearing since the slope of the rear wall of the drum deflects the load as it lifts and drops during rotation.
Horizontal axis: load distributed more evenly to front and rear bearings. Rear bearing does not have to take thrust load. Front seal more critical since water line may come up to seal level more often. More vertical pounding with small loads and more adjustment needed to get up to spin speed without offbalancing.
Washing action should be comparable as long as length of drum does not exceed diameter and inclination is not more than about 15 degrees, assuming all other factors equal including blade design. However, speed of rotation could be a factor; inclined axis drums should rotate more slowly to allow lift-and-drop action and avoid unproductive "carry-over."
Horizontal drums should be possible to handle slightly larger loads than inclined axis drums, because the critical variable is how high the load is relative to the axis itself. The higher the load line above the axis line, as measured when the drum is at rest, the less scrubbing action will occur and the more the load will tend to rotate with the drum. In the latter case, the washing action will be primarily due to water passing through the clothes rather than due to mixing & scrubbing action and flexing of fabric.
These design issues are not very far different from those related to concrete mixers and other revolving-drum materials handling equipment. In that regard, a concrete mixer of 3-1/2 cubic foot capacity (standard contractor's size) has to handle a load that weighs 525 lbs., and does so without complaints for ten to twenty years (typical useful life) or more. Therefore it's safe to say the design issues re. bearings and longevity have been solved for almost a century; the issues re. doors & seals have been solved since Bendix; and the issues re. speed of rotation relative to load characteristics have also been solved long ago.
The two issues that remain most open to innovation are the design of blading to handle different types of load composition (e.g. a load of white underpants & socks is radically different than a load of jeans & towels), and issues related to maximizing the cleaning ability of smaller quantities of water. These are the points on which you can expect to see noticeable advances in technology in the years ahead.
When I was a kid we had a dryer that had one paddle (as such) and two "bumps" (that resembled speed bumps running the length of the drum; and these three elements were not equally spaced (i.e. not at exact 1/3s of the circumference). When I first saw this I thought it was odd, but it quickly made sense because I figured the spacing would tend to counteract "carryover" of the load at a given rotation speed, and the combination of paddles and bumps would produce a combined rolling-over action with lift-and-drop mixing of the load. I suspect something similar would be useful in washers as well as dryers.
