What is your tax situation? If you are planning to FIRE at some point soon, or at least work less, you may want to look at minimizing your current tax burden (ie contribute to 401k even without a match, IRA, etc). If you think you'll make *more* money in the future, on the other hand, maybe that's not as important. You might want to read the "how to write a case study" sticky and edit your post - there's a ton of detail missing to answer this seemingly simple question.
That said... if the 5.9% rental is a solid property (for cash flow, for appreciation, for both, etc) then absolutely kill that mortgage first. In theory you could do a *tiny* bit better in the stock market but that 5.9% is a completely guaranteed return. Putting aside diversification concerns, it's probably a no-brainer to pay that one off as fast as possible. I'm assuming you're in the US - you might also look at refinancing it since you should be able to easily get ~4% (or less) and 15/30 year amortization on that 5.9% loan.
If you can refi it, then I'd just dump everything available into the index fund of your choice, in whatever way works best for your tax situation since you have plenty of RE in your portfolio already.
-W
