An extra payment on a loan is, broadly speaking, a known-return, risk-free investment. (That the return on the investment is in reduced costs going forward instead of increased revenue is basically immaterial, assuming you have sufficient cash flow to handle either situation.)
We can't know what the interest rates will be like going forward, but we can know what they are today, because you gave us those numbers in your question.
Quick now: Given the choice between a known return of 3.7% annually and a known return of 7% annually, with identical (and extremely low) risk, where would you invest your money?
By putting the $15k toward the $14k loan, you free up $140 per month and have $1k left that you can put toward the $30k loan, which will reduce your payment term by $1k / $260/month or about 4 months. You will be debt free in 14 years 8 months. You pay $14,000 instead of $16,800 on the $14,000 loan, reducing the total cost of the loan by $2,800, and reduce the cost of the $30,000 loan by four months' worth of interest which is about $175 (so the $30,000 loan ends up costing you something like $46,600 instead of $46,800).
By putting the $15k toward the $30k loan, you cut the principal of that one in half. Assuming that you keep paying the same amount each month, you will reduce the payment term by 7 ½ years, and will be debt free in 10 years (because the $14,000 10-year loan now has the longer term). Instead of paying $46,800 for the $30k loan, you end up paying $23,400 plus the $15,000 = $38,400, reducing the total cost of the $30,000 loan by $8,400 while doing nothing to reduce the cost of the $14,000 loan.
To a first order estimate, using the $15,000 to pay off the $14,000 loan in full will improve your cash flow in the short term, but putting the money toward the $30,000 loan will give you a three-fold better return on investment over the term of both loans and nearly halve the total loan term, assuming unchanged monthly payments and unchanged interest rates. That's how powerful compounding interest is.
