small business type, ownership, and taxes

[TheBeeKeeper]

besides our full time jobs we have a small business my husband and I started a couple of years ago. It nets about 20K/year and we're slowly growing.
When I opened it the easiest thing seemed to be sole proprietorship, and since I handle all the bureaucracy I just registered it under my name.
I'd like to invest all the income from the business, and minimize taxes
We are both close to maxing 401Ks through our employers, and now I've started looking into tax efficient way of investing the business income.
My questions are
1. is it the best thing to remain sole proprietor? or change to partnership with spouse, or a corporation?
2. what is the best investment vehicle for our scenario? We are in the 15% tax bracket after all combined income (work, business, and rental properties) , and for 2015 we paid only self-employment tax on the business income.

TIA!

[DavidAnnArbor]

If your business income is in partnership with your spouse, then you get to sock more of the money away in a Solo 401K, whether it be through an employee salary deferral (which you have to combine with the employee deduction you already make at work to make sure you don't go beyond $18K for the year per person) or through the employer deduction.
If your business income is only $20,000 I don't think it makes sense to change to a corporation to avoid the FICA taxes because as a corporationyou have to pay for Unemployment Insurance, FUTA and SUTA, plus the burden of having to do payroll checks. 

[TheBeeKeeper]

If your business income is in partnership with your spouse, then you get to sock more of the money away in a Solo 401K, whether it be through an employee salary deferral (which you have to combine with the employee deduction you already make at work to make sure you don't go beyond $18K for the year per person) or through the employer deduction.
If your business income is only $20,000 I don't think it makes sense to change to a corporation to avoid the FICA taxes because as a corporationyou have to pay for Unemployment Insurance, FUTA and SUTA, plus the burden of having to do payroll checks.

does this mean if both of us max the employer 401K then we cannot put more in a solo 401k?

We're at 20K/year income, but planing to grow in the next few years

[Vilgan]

You get 18k per person total, regardless of where you are deferring it from so if you are doing 18k at work you can't also defer 18k at home. However, there's also profit sharing which caps way higher so you can put 20% of the sole prop income away pretax as profit sharing which is probably worth it. I'd recommend opening up a free solo 401k with Fidelity or Vanguard.