If you are under age 50, the 2014 cap is $52K for either a SEP or a Solo 401K, the difference is that you can reach the cap with a lower income level with a Solo 401K than with a SEP. If you are 50 or older, the total Solo 401K cap is $57,500, the SEP stays at $52K.
SEP: Max employer contribution is 25% of income, so a person would need to earn $208K to contribute $52K. No employee contribution.
Solo 401K: You contribute both as the employer and the employee. As the employee, you can contribute up to $17,500 ($23,000 if over age 50). This can be every dollar of the first dollars you earn. Then the employer contribution can be up to 25% of your total pay, too. So, you can reach the $52K max total contribution after earning just $138K.
It's not quite as simple as that for a couple of reasons (esp Self-employed tax deductions, etc), all numbers above are approximate, etc, but that's the basic idea. The impact of self-employment taxes, the 1402(a)(12) deduction, etc can be considerable. The calculator
here will consider these things and show your max contribution for a SEP, Solo 401K, Simple, etc. For all income levels, the Solo 401K will allow a higher contribution amount, until you hit the cap (and, the cap is higher for the Solo 401k when you are over 50). For example, the calculator shows a 51 year old with earned income of $150K can contribute over $51K to a Solo 401K, but just $28K to a SEP. For the 51 YO at $200K earned income: Solo 401K =$57,500 (capped) , SEP = $38K
If you are earning enough to hit the $52K cap using your SEP and you are under 50, then maybe stick with that. But if you are over 50 or if you expect your earnings from the ER gig to be less than approx $275K (maybe devoting more time to the urgent care biz?), then the Solo 401K might be. . . wait for it . . . just what the doctor ordered.
I'm just a know-nothing on the interwebs, hopefully your accountant has already explored these options with you.