For small volume, either is fine.
Sendmail is the most versatile MTA around, but that often comes at the price of learning it's very cryptic configuration syntax (much can be done with the m4 config, but for some things you have to dig directly into sendmail.cf rulesets). On the plus side, you can do pretty much everything with it, such as routing email based on Message-IDs or such ;-)
Postfix can do all the standard things too, and is much easier to understand and configure. Unless you want some very unusual features (see above), Postfix should get the job done.
There is also at least one other free alternative, namely
qmail. It's designed for speed and security and is very lightweight, and although the configuration is a little unusual, it is easy to comprehend.
