[Exim] Flat files vs DBM

I am wondering the list's opinions on using a flat text file vs a DBM hash
for the local-domains setting. I plan to have a considerably large number
of "local domains", and am wondering which will provide the fastest access,
both for Exim looking up entries and for myself (or a script, etc) to be
able to modify the contents of the file.

Thanks.

Trevor Sky Garside
trevor@???