When is investment loan interest not tax deductible?

[Richard Brightwell]

I'm not sure what my options are... I just bought a $171K SFH, rental-A, using 75% mortgage and 25% cash down. I spent $18k fixing up rental-A, rented it, and am getting good cash flow. I'd like to pay myself back the 25% down and the $18K. I have a $200K SFH rental which is debt-free, rental-B. I also have a 401k I can borrow up to $50K from. What are my best options? Should I:

1) Cash-out refi rental-B to repay myself? Should I get a little extra (higher LTV) for the next investment? Will the interest be deductible?
    2) Take a $50k loan against my 401k and settle for getting most of my money recouped? Is the interest deductible?
3) Should I get a HELOC on rental-B to repay myself? Then have the HELOC for future investment opportunities? Is this deductible?
    4) Would it be better if I used a corporation in the future? Does that make all the above debt deductible?