Solo 401K vs SDIRA? Which is a better tool for flipping?

[Cass Lowrie]

I fix & flip homes. In 2019 that business will profit about $250K. I have an LLC for buying properties and another LLC that holds a contractor's license. (Company A buys the property and Company B renovates it.)

My wife has a 401K from a former job.  It only has $25K in it but I want to get that off the sidelines and invested into our fix & flip business to try and shelter some income taxes.  

I first thought a SDIRA was the correct choice, but the more I read about them, the more restrictive they seem.  (Seems like I would break tax law without ever meaning to!)

Now I'm reading about Solo 401K's and they are starting to sound like a much better tool.

Asking for any advice from the seasoned PRO's here in BiggerPockets.  There is a tremendous amount of experience and wisdom here and I'm hoping to learn from that.  Seeking advice and very thankful for any that comes my way.

What's your opinion/experience with the investment vehicles?  Which is better for fix & flips?

Thank you in advance!!