As I shop for a new espresso tamper, I see that there are a dizzying array of options for the shape of the base (i.e., the shape of the part that contacts the grounds). Some shapes include convex ("Euro" more curved than "American"), flat, rippled, or hybrids of those -- see some examples at Reg Barber. (I'm also aware that there are many shapes and sizes of the handle, and differing sizes for the base; I'm not asking about those.)

In part spurred by this recent question on the essentials of tamping, and other questions tagged tamping (which are mostly about pressure), I got to thinking that the shape of the tamper could have a significant impact on the result.

There also seems to be a fairly large amount of personal preference or opinion in this realm. I can't find any proper comparisons or trials; at best I can find conversations about various preferences (e.g., home-barista including this poll, coffeegeek, coffee forums, barista exchange). A few patterns emerge:

• curved bottom provides a greater margin of error for avoiding the "donut" effect of water channeling around the perimeter of the portafilter;
• curved bottom could over-compact the center;
• ripple shape is pretty, is used by some pros including winner of a competition in 2009, but has no demonstrable benefit, and is marginally harder to clean.

I hope I'm not spurring a sectarian flame-war by asking this; so I try to ask as objectively as possible:
Is there any objective benefit for one tamper base shape over another?

For example, side-by-side trials; experienced use beyond preferences; ...