Should I pay off my mortgage or invest?

"Better" is all relative. It's interesting because I am in the process of writing an article for my blog about this very question - i.e. should you focus more on investing/saving or accelerating debt payments.

As a very general guideline, I usually suggest that you not worry about accelerating the payments on debt at 3% or less; on debt at 6% or more, accelerating it at least some (the higher the rate, the more you should consider trying to accelerate it); for debts between 3% and 6%, I call that the "gray area" and it depends.

Additionally, some of that will depend on your age, your investment time horizon, etc. Since your mortgage is 3.75%, if you have a long-term time horizon for the investments, then (without knowing quite a bit more about your overall financial situation) if it is between these two options, I would tend to say that you should focus the majority of the disposable income on investing. The main reason is that long-term, you should be able to easily get a better return than 3.75% with a properly allocated and well-diversified portfolio. It's no guarantee, of course, but history would suggest that long-term, unless you are very heavily invested toward the fixed income side, your return should be a fair bit higher than 3.75%.

However, that is not to say that you can't do both. For example, if you had $500 per month extra, you could put $100 toward the mortgage and $400 toward investment; or whatever split you want. Though as I said, without knowing more, I would probably weight it more toward the investment side.

Finally, I'm not accounting for the value that comes with becoming debt free as quickly as possible. There is a definite psychological value to that, but it varies from person to person, and can even vary for the same person depending on their present situation. So, it is very difficult to quantify even in the best situation, and impossible to quantify in this situation since I don't know how you view debt, etc.

Hope that helps some and best of luck.