Almost always IRA. Roth IRA allows your investments to grow without taxes--including any taxes you may incur from capital gains or dividends--and you don't have to pay taxes on that money when you pull it out either. Sure, the discount on the ESSP is nice, but depending on how long it takes to vest (I'm assuming you can't turn around and sell it immediately), you may end up with too many of your assets in one place, which could be really bad if the stock price goes south. That, and you don't get the tax advantages of the IRA. My $.02.