> purchasing one system over the other?  For that matter, given a simillar
> scenerio in 1985 with a VAXstation 2000 vs MicroVAX 2000, why would I
> consider purchasing one over the other?

Cannot answer the 3100 question but the VS2000 ones I can.  The Microvax2000
and VS2000 were really the same board.  Options were the primary difference.

Option list were disks RD31,32,51,52,53,54, tape (tk50), color vs mono 
console, floppy rx33, NI lance, memory.  

Most of the microvax2000s had rd53, 4meg, mono and lance.  The VS2000s were
color, rx33, rd32, 2meg ram and lance.  There were other variations.  I know 
of one that had 6meg, color, lance, RD54 and TK50(external) ran VMS with 
fair perfomance for a dozen users with eithernet access.

Those were the one most seen inside DEC with the group I was in back then.

***opinion Since the CPU variations are not the most significant difference
   the areas needed the most attention for vax systems in general are the
   devices.  It's the devices for the most part that differentiate
   the vaxen as most cpu comparisons would be like comparing 386 and 486
   family parts, They all run mostly the same code(fpu aside) but
   performance differs greatly. As an aside the backplane used also 
   differentiates them as well being there were unibus, Qbus, BIbus systems
   never minding the bounded systems and afew I may have forgot.

Allison