You have three choices for the funds in your old 401(k) plan. The two you mentioned (leaving it where it is or rolling it over to your new employer) and third, rolling it over to an IRA. The best option for you would depend on several different factors, but generally . . . .
Focusing on debt payoff can cost you hundreds of thousands of dollars. Debt payoff strategies abound on the internet, and many mainstream financial gurus have built an empire on the common-sense approach to focus your financial efforts first on paying off your debt. The math, however, doesn’t work out in their favor.
The Stock Brokerage and Insurance Industries are fighting hard against the Department of Labor's Fiduciary Rule, which would require their advisers to put their client's interests first and disclose conflicts of interest. The industries got the 5th Circuit Court to agree that, unlike Registered Investment Advisers, stock brokers and insurnce agent are just salespeople
Congress recently reversed rules which allowed states and cities to create Auto-IRA programs. Although I like the intent and am a big fan of encouraging retirement investment, the program wasn't ideal. A lack of ERISA protections, potential for poor management, and poor investment options all mean I won't mourn the death of the Auto-IRA.
Historically, there was a "three-legged stool" of retirement which provided Americans with a comfortable and safe retirement. The stool, however, has been eroded away, leaving many without sufficient income in retirement. Making small changes now can net a massive improvement in your lifestyle in retirement, and may provide you with the option to retire earlier or wealthier.