It so happens that cree has launched a 50w equivalent led MR16.

"It so happens that cree has launched a 50w equivalent led MR16."

Yes, I saw that but aren't MR16 inherently more prone to failure due to the small size and shape of the bulb? I've seen reports of the $30 SORAA bulb failing right away.

Not necessarily. It depends on the components used and design of the heat sinking.

OK. Since I need to replace my fixtures anyway, if you were choosing between a PAR 20 fixture and a MR16 fixture for the purpose of display lighting, which would you choose? I'm more worried about function and reliability than the appearance of the fixtures.

If the only choices are PAR20 and MR16 -
A PAR20 bulb since it will most likely be line voltage and does not require a power supply.

Thanks David.

"If the only choices are PAR20 and MR16 - "

David,
Now your last answer has me questioning: is there something better I should consider? Also, is there a particular fixture style that would be better (or worse) for retrofit LED bulbs?

Without knowing what is being lit and other considerations (initial / operating cost, expected service lifetime, building limitations, portability to a new location ...), it would be difficult to say if something else would be better.

If you're open to considering alternatives, lighting vendors such as
CSL lighting, Philips/ Color Kinetics, pixi-lighting, nu-world distribution, ...
have a wide range of products to look at

The link below has some track lighting kits built with LEDs.

Here is a link that might be useful: some choices

Thanks. I'll check those out. I've been searching for native LED fixtures but everything I found was $200+ per 50W fixture which was more than I wanted to spend. I'm lighting frame displays. I want something that will do the job reliably and doesn't look cheap but the fixtures don't need to be the focal point of the room.