FYI, the D3100 does not have a external microphone jack either.
If I were starting out in photography right now, I would buy the D7000. If you are planning to "seriously get into photography", once you start using the D3100 or the T1i for a bit, you are likely going to start wanting some of the features of more advanced camera bodies. However, if you are on a budget, or you are unsure of how much you will get into photography, then these are both cheaper options that will serve you well in the short term. I currently own a T1i and I love it.