Had one invested $478/mo in index funds for those 21 years they'd have $1,000,000 - starting with nothing!  

Stock index funds are one option but a little worried about that being at it's peak if not near it's peak.

Just in a checking/savings account or possibly in index funds or something like that?

Since I wouldn't be able to access this money till the age of 59 1/2 I am wondering if it would be better for me to lower the amounts that I am putting into these accounts to either put some into index funds or invest in more real estate.

If you invested that $600,000 into whole market index fund like VTI, you would see the value increase over $50,000 per year.

My personal preference is to look at metrics which take the opportunity cost of capital into account such as internal rate of return (IRR), net present value (NPV), or profitability index (PI).

@William MurdenIf there is a broker involved they might have looked at the supply index but I've rarely seen brokers do that and even then I'd do my own due diligence on it.I recommend picking up a copy of the Self Storage Almanac from Mini Storage Messenger.https://www.ministoragemessenger.com/product/2018-self-storage-almanac/As far as doing your own research on the supply index, @Mark Byrge is correct.

Appreciation seems too close to the same returns as index fund. what kind of appreciation have you seen?

We plan on doing other boring investments (mutual/index funds, 401K etc) for a bit before jumping into RE, but I want to continue to learn as much possible between now and when that first move happens.

I personally would rather try to use OPM and put my own money to the best possible use such as doing private lending, investing in syndication or note fund, or even index mutual fund... all of those would yield me over 10% return.