I took the lump sum at the end of last year. I had that money rolled into a Schwab IRA. I retained my money in the Ford SSIP as well as a 401k that I had at GM from a former life.
Initially, I was going to move my 401k from Ford to Schwab, for ease of bookkeeping etc. I'm glad that I did not do this.
Ford and GM (as well as many others) offer comingled or pooled funds. These funds have lower fees. For the Fidelity Growth Company Fund, it is close to 0.5% lower for the fund offered through Ford, v the retail shares at Schwab. So, for every million you have in your 401k, you will save $50k/yr keeping it at Ford. (assuming other funds are similar - I'm going to track this down.)