A variation of this question is a staple of the forum: invest or pay down a mortgage?
Close, but not exactly. In this example you free up $6,000 a year (at $500) a month but pay $1,500 (@300k loan ?) a year to get this. In the early years out, you would need one heck of a return OVER the 2-3 percent interest paid on your cash flow to get that. You pay the higher rate on the entire loan balance but can only earn interest on you cash flow savings.
Phrased differently, if you pay down the loan with say $75,000 or whatever, that entire amount is immediately investable.
IMHO if you are using leverage or home loans or whatever to invest, it matters what you invest in. Using borrowed money means you probably should probably be more conservative with your investments. If you have a moderately aggressive portfolio depending on age .... but lets say 70/30 (stocks / treasuries & cash) ...it would mean 30% of your investments are earning less than the rate on your home. You could get more aggressive with bonds ... more aggressive with stocks ... but doing so with borrowed money adds to risk ... doing so with borrowed money is where some run into trouble..
One poster on another thread was something like 90% stocks or something like that, but with no mortgage at all,the poster was, as I recall, late 40s ... to me this makes intellectual sense for aggressive investors (though not for me personally). It also makes sense to me if you have a mortgage to have more bonds.
The OP is maxed out in tax deferred, so all investments would be taxable ... dividends at a high rate ... which makes generating that extra $1,000 a year that much harder, particularly in the early years. Given this, the 15 year loan makes sense (and perhaps tweak your tax deferred to be more aggressive).
I do not think there is a choice that is knowably wrong ahead of time.