horizontal partitioning or multiple tables?
I just ran across this old question.  In general, administering 1 table would be easier than administering 10-100 tables.  Imagine trying to ALTER TABLE across 100 tables!

Just because you have millions of rows doesn't mean you'll need multiple servers.  Good queries, good indexes, good disks, etc., are all performance items.  Fewer servers are also easier to maintain.

What did you end up doing?