“If you don’t have any traditional IRA (say as the result of a rollover from a previous 401k or 403b), SEP-IRA, or SIMPLE IRA, you are in good shape. Skip to step 2. If you are married, please note IRAs are owned by one and only one person. Each spouse should look at his or her IRAs separately.
If you have a traditional IRA, SEP-IRA or SIMPLE IRA, and you don’t mind converting all of them to a Roth IRA now, also skip to step 2. You will have to pay taxes when you convert those IRAs.
If you have a traditional IRA, SEP-IRA or SIMPLE IRA, but you don’t want to convert them and pay taxes at a high rate just yet, rollover almost all the pre-tax money to an employer sponsored retirement plan: 401k, 403b or 457. Most employer-based plans accept incoming rollovers.”
Backdoor Roth contributions are another reason for choosing a Solo 401(k) over a SEP-IRA:
“IRS form 8606 which handles Roth conversions and IRA basis looks at Traditioal IRa, SEP-IRA, Rollover IRA, SIMPLE-IRA when figuring out the taxable and non-taxable portions of conversions and withdrawals. TSP, 401k, 403b are not considered by form 8606.”
http://whitecoatinvestor.com/retirement-accounts/backdoor-roth-ira/ “Now the catch. If you have any other traditional IRAs, this can really screw up the conversion due to the pro-rata rule. This basically says that if you have both pre-tax traditional IRAs (includes SEP-IRAs and SIMPLE IRAs) and non-deductible traditional IRAs, you must convert them “pro-rata.” So if you have a $20,000 pre-tax IRA and a $5000 non-deductible IRA, and you convert $5000 of it to a Roth IRA, you’ll end up having to pay taxes on $4000 of that conversion. You only want to do that if you’re in a low marginal tax bracket (which most physicians aren’t.) So you must somehow find a way to isolate that non-deductible IRA. The easiest way to do this is to roll it into a 401K if possible. (Most 401Ks allow this.) Traditionally, financial advisers have recommended that you roll your 401Ks over to IRAs every time you change employers to take advantage of lower fees and better investment choices. The Backdoor Roth IRA shows at least one reason why it might be better to leave the money in the 401K.”