Need Advice: Self-directed IRA vs. Solo 401K

[Adam Meadows]

Hello BP,

I need some help regarding how to proceed on determining a SDIRA vs. Solo 401K. I've got about $100K in a 403B plan (equivalent of 401K for non-profit organizations) from a former employer. I am currently an employee of another healthcare organization and have a separate 403B through them. My wife and I have an LLC for real-estate investing purposes, and we are both managers and members. So here are my questions:

1. Am I even eligible for a Solo 401K if I am still employed in a W-2 position?

2. Which is a better vehicle for real-estate investing? Pros/Cons of each? For instance, I've read you can borrow against your Solo 401K, but not with a SDIRA.

3. To what extent do you diversify within a given account? Meaning, would it be prudent for me to keep some part of the $100K in stocks, or commit the entire sum to real-estate investing? I realize my audience may be slightly biased here :-)

I also know it's not a super large amount of money in the grand scheme of things, but I want to be strategic about how I allocate it. Thanks in advance for your input!