Those devs who spend time and effort targetting PS3 and trying to get the most from it will be the devs that show the best the hardware can do, and that's where you'll see which platform is 'most powerful' - when the software isn't the limiting factor.
Software will nearly always be the limiting factor between these two consoles. If you were comparing Wii and PS3 then yeah, hardware is the main factor. Between PS3 and 360, though, it's all about software. A factor of 2 is visible in graphics (be it with RAM or speed), but very hard to identify with the CPU.
Take joker's example of cloth simulation in NBA. If you want to do it right without it looking like a springy mess, you need an implicit solver using conjugate gradient, which generally scales like n^1.5 (n is the total number of vertices). Now, you look along the edge of a player's shorts and see 10 polygon edges on 360, and 13 on PS3. The total # verts in the shorts is then 1.69x greater on PS3. The processing time is 2.2x greater. So assuming Cell can do this algorithm this much faster on top of the extra load of culling for RSX assistance, how much of a difference is that? Better yet, what if 360 had the same number of visible vertices, but had fewer control points and simply interpolated? How visible is this double computation?
GT4 is another example. Simple lighting model, but perfectly tuned. Another dev team does twice the math per pixel but still can't get the cars to look as realistic.
For different games, you cannot tell which hardware is more powerful because it's all about the software. For the same games released at different times, simple changes again can make a huge difference. Enable AF and disable normal mapping (!), and voila! 9/10 people think Oblivion's ground looks better on PS3 because it's more capable or powerful. Oblivion is one of those games that really needed amazing hardware to run well, but it really didn't look much better than other games. They really didn't put much thought into incremental cost/benefit analysis before the 360 release, again proving my point of how judging hardware from game output (and thus vice versa) is silly. Even simultaneous releases are usually done by different dev teams, so again we have little idea of which is more powerful.
XB360 started nearer it's peak abilities due to an easier development environment. Because of this, it's advances won't be so pronounced. PS3 started slower as devs are trying to get their head around it's unique processor arrangement.
That's an assumption that's ignoring everything we know. RSX is based on the same graphics technology we've seen for many years, and Xenos is the radical one. Why should the former be harder to work with? Most of the differences we're seeing are graphically related. Available RAM affecting textures, vertex setup and shading, render BW, etc. Unfamiliarity and "peaking" are not huge issues here.
We're not going to see RSX suddenly become twice as good as Xenos regardless of backface culling. Even for fine-grained frustum culling, quadrupling your packet size will only slightly increase GPU load while making CPU load one quarter. Thus it's fully applicable to the 360.
I maintain that CPU power alone has little effect on the quality of a game if a developer is clever about it (unless we're talking about 5-10x). You can't see what's going on behind the scenes, so if you tailor your load to your system then the perceptible impact is minimal, especially when your graphics load must be the same or less.
Sony will have some fantastic games for the PS3, but they will be great for the same reason they had great games on the PS2. It's all about the software and art.