Do both!
1) That's a decent ESPP. 15% discount is really good. With a 1 month holding period, your annualized return on the sequestered money is something crazy. So take it, and sell stock every 30 days, immediately once the stock has "aged" 90. Move it to checking afterwards and live off of it, if you have to. If you have any kind of emergency fund, you're plenty liquid enough to bootstrap this and give yourself a ~$3.5k-$4.5k raise.
Your emergency fund needs decrease somewhat because you could always suspend ESPP participation to dramatically increase cashflow. (Once you suspend you'd still be getting stock sales from your 90 days "pipeline", but you'd also stop deferring so your paycheck goes up.) Probably still want to put some of that "raise" into replenishing the e-fund, though.
2) Now that you're living partially off of stock sales, doesn't that create room in your paycheck to increase AT 401(k) witholdings? Yes it does. You created even more room to do that than you deferred into the ESPP. (Because the discount is turning into cash in your pocket.)
If you insist on choosing one or the other, definitely take the free money. (ESPP.)