With a SEP IRA, you can only contribute 25% of your net profits (less 1/2 FICA tax) up to some limit (I think it was 53k, as Papa bear said). This sucks.
I'd recommend a SIMPLE IRA if you currently make and forsee making less than $12.5k from this LLC. This is because the reporting requirements for a SIMPLE IRA are much simpler (haha) than that of a solo 401k.
With a SIMPLE IRA, you can contribute 100% of your first 12.5k in net profits (less 1/2 FICA tax)
However, a solo 401k lets you contribute 100% of yoru first 18k in net profits (less 1/2 FICA tax) as an employee, and 25% of your profit as an employer (less 1/2 FICA tax). (The SIMPLE IRA only lets you do 3% as an employer). So if you would max out a SIMPLE IRA, definitely do a solo 401k instead.
If you want a recommendation for the SIMPLE IRA, I opened one with Schwab and it's pretty good. No maintenance fees nor minimums. You can trade Schwab ETFs for free. They have a pretty good selection of index ETFs. I personally buy only SCHB in that account (tracks Dow Jones broad market index).
The one thing I don't like about it is you can't make contributions online. You either have to fax or mail in a form. Thankfully the contribution itself can be done via ACH, once you've linked the bank account.
I didn't go with Vanguard because they don't have admiral shares available in their SIMPLE IRA or 401k. the SCHB ETF has an ER of 0.03%, which is even lower than Vanguard's VTI (though it should be noted they track similar but not identical indices)!