I'm also concerned about this because I'd like to make 3D games for the web. It's hard to find the perfect solution or just get all the advantages with one technology. There's also the O3D from Google, however it also requires a plug-in. It's based on WebGL. It looks powerful and the graphics are quite beautiful. Personally, I'd tend to go with Unity3D even though the penetration is not 100%. Flash is not 100% too, it between 90%-95%. I believe if you sell your game correctly, many people will install the plug-in. At the moment, I think 3D is still immature for the web and making comprises is the only way to use 3D. Probably the web version could have lower performances. Web browsers are not made for performances. A downloable version could be used by people who want more performance. With Unity3D, it seems that can be done more easily.