I have a IRA, HSA and SE401k to maximize tax-deferred investing.
The choice between the SE401k and SEP IRA will depend on how much you have to put away this year and whether you expect that to change in the future. The SEP IRA caps at 25% of income. The SE401k caps at 25% of income plus 18K.
Sometimes you hear that the SE401k is expensive or time consuming to open. It was a couple page document when I opened mine with Fidelity. I called them with some questions, which they helped walk me through. It took maybe 1.5 hours total and there were no fees. For the ability to sock away another 18k/yr tax-deferred, it's a no-brainer.