Usually the biggest reason we see for recommending leaving a 401k is for those closing in on retirement in their mid 50s, and they might need to use some of the 401k funds prior to age 59.5. There are ways to get at IRA funds prior to that without penalty, but it's a little simpler from the 401k.

A con is that depending on the plan there can be high fees on the 401k that you may not be aware of.

Great article on the fees inside 401ks that sometimes are passed on to employees:

http://www.filife.com/stories/not-all-pay-share-of-revenue-sharing

Well, since it started out as Roth money that means you already paid taxes on it. So, it would make sense to roll it over to a Roth IRA to keep that tax free status when you want to withdraw. I'm sure the investment company you choose will help you with that decision, though.

Wow, roll it into an IRA, that's a great idea.

Follow up question: it was a Roth 401k, does that mean I can turn it into a Roth IRA?

You really can't go wrong with fidelity's IRA

http://www.filife.com/reviews/fidelity-simplestart-traditional-ira

Pro for separate = diversification...you probably have a different choice of funds between your old job's 401k and your new one's. The cons for keeping it separate are that you may forget about it so you won't check your investment allocation, or you forget that you own it completely, or, there's always the VERY remote possibility that your old company will go under and you could lose your retirement fund. I think you should roll it over into an IRA with a reputable investment company independent of your new 401k, so you keep the benefit of diversification but you lose the risk of forgetting about it or losing it.

It would make sense to compare the investment options in the two plans; if the old plan has poor fund options and high fees, rolling into your new plan (assuming it has a Roth 401(k) option) or a Roth IRA is a good idea.