retrieving Packages/Sources files from mirrors

Hi,

I maintain the service bootstrap.debian.net which so far was using
snapshot.debian.org to retrieve Packages and Source files for all architectures
of Debian Sid daily at midnight UTC. I used snapshot instead of a mirror
because I want to:

 - be able to retrieve the exact same Packages/Sources files at any later point
   in time to investigate why a run failed
 - avoid that some Packages/Sources files for different architectures come from
   two different mirror pushes (in case I happen to download just at the time
   the mirror updates)

On the other hand, using snapshot.debian.org has the disadvantage that I'm
stressing its bandwidth for something that can maybe achieved using normal
mirrors.

For example if I download from a normal mirror and additionally store the
Release file, it should be easily possible to verify later that I'm using the
correct snapshot from snapshot.debian.org.

The harder problem is to make sure that I obtain the Packages/Sources from all
arches for the exact same mirror push. My internet connection is not the
fastest, so downloading all Packages/Sources files will take a while. Is there
a way to make sure that I do not start the download when a mirror update is
imminent? Or the other way round: is there a way to know the time of a mirror
push so that I can download right after it is done?

For my purposes, is there a difference between which mirror I am using? Using
mirrors like http.debian.net might make my situation worse because it might
redirect me to different mirrors with different state between individual
requests, right? On the other hand, if I use, say, ftp.us.debian.org then those
requests are also distributed over several servers, right?

Please keep me CC-ed because I'm not subscribed.

cheers, josch