@Ethan Koyle,
You’ve got pretty interesting sources of financing at your disposal. On the home equity front, Rafael Gonzalez is right—you can take a second-position mortgage, like a home equity loan or a home equity line of credit, without having to refinance your first mortgage.
That said, any second-position loan you take out will still be priced at prevailing interest rates, so just a heads up.
The 401(k) loan could be an interesting option, since you’re ultimately borrowing from yourself. One thing to note here is opportunity cost: by borrowing against your 401(k), you’re forfeiting investment opportunities in the market that could potentially yield returns in excess of the interest you’re paying yourself.
In my opinion, however, the biggest risk of borrowing from your 401(k) is the risk of defaulting, either because your deal went wrong—or simply because of clerical error. If you default, the entire unpaid balance of the loan will be considered a distribution, and you’ll pay income taxes plus an early withdrawal penalty.
As for which is better…I think you’d have more luck asking your CFP or CPA. This is just some food for thought!