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 . . . .
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.
If you don't currently have a Roth IRA, open one today. Adding a Roth IRA to your retirement plan provides you with a wide variety of benefits, including tax diversification, access to early retirement, increased limit on retirement contributions, and greater control over your retirement investment options.