UP2000 vs DS10 for beowulf?

[David Bremner]

I am looking at two different configurations of 600Mhz, 2M cache alpha
21264w

   6*2 processor  UP2000, 512M RAM

or

  12*1 processor DS10L 256M RAM

both options include high speed networking (myrinet in one case,
wulfkit in the other; but possibly I could swap).  Both will run Linux.

The pricing works out approximately the same.  Is there anything 
obvious that favours one of these configurations over the other?
If the answer is "it depends", what does it depend on?

One thing that is a bit worrying is having two processors talking
through one PCI bus to the high speed network.

The other is that I understand the UP2000 memory subsystem is not as
good as the ds20, and apparently can't provide the same bandwidth to
two processors. (see
e.g. http://www.dl.ac.uk/CFS/benchmarks/compchem.html, for anectdote,
but no numbers.) Does anyone have two processor stream numbers for 
UP2000s?

Clues for the clueless?