Consider diversification in terms of your entire portfolio (that is, all the investments/stocks/accounts you own) while also looking at taxable/non-taxable and stocks/bonds/real estate allocations. You have real estate exposure but no stocks/bonds that I could find in a quick glance over the posts. The real estate equity can be considered non-taxable in that any gains are protected from taxes (up to a point).
Your income is low enough at 38k that you are in a lower tax bracket. If you put this extra cash in a Roth, you pay ~25% federal and whatever your state rate is on the contributions only. It then grows and in 35 years it is all tax free. Since you are young, you can afford to be aggressive with this money by investing heavy in stocks. Assume a 7-8% rate of return far exceeds your mortgage rate, especially considering the fact that the mortgage has tax advantages. Plus you can start this immediately and it doesn't seem that you have any retirement savings as of yet (time to get moving on this area... NOW). If you get in a pinch (and I mean a serious pinch), there are rules for tapping the Roth before age 59.5.
If you were to save for an investment property, you'll have to run the numbers on your own as I don't know enough about your specific situation. Consider the fact that any investment property will carry a higher interest rate on a mortgage as the risk associated is higher. If you were to setup the property in such a manner as to protect yourself from liability you are looking at attorney's fees, incorporation costs (LLC?), accountants fees (cause you aren't going to want to do the taxes on that bad boy), maintenance, and property taxes. What kind of income would this property then generate on a monthly basis? Also consider that you won't be able to buy this property immediately, as I would assume you would need 20% down to get any property that you won't be living in.
If your concern lies in the fact that properties are cheap now and you want to get in an investment now, take a little faith in that the housing market isn't going anywhere any time soon. In the large scale, people just don't have any money to spend on a house. Credit is still tight except for those with excellent balance sheets. The opportunities for investment will always be there. The one thing that won't be there for you is time.
My $0.02, invest in the Roth, and do it TODAY. In 35 years, consider how liquid a house or investment property is vs an account with money in it.