Keep in mind that since you don't have a lockup period with the ESPP, that money isn't set aside to nearly the same extent as it would be if you put it in the 401(k). Therefore if you really can't do both right now, start with the ESPP. After the first six-month period is up, sell your ESPP shares, increase your 401(k) contributions by 10%, and cover any difference between your paychecks and living expenses with the money from the ESPP shares you just sold. Continue contributing to both the 401(k) and ESPP indefinitely.
Skipping 10% contributions to the ESPP is like leaving 1.5% of your salary on the table. Don't do that, especially once you're already getting the maximum 401(k) match!