File the flight plan either way. The VFR flight plan is the formal arrangement between you and the FAA that gets people to hike through the mud in the woods at night in the rain looking for you. VFR flight following is more impromptu.
This is not to say that your sudden disappearance during VFR flight following won't concern anyone. The 7110.65 makes mention of this very eventuality...
10-2-5. EMERGENCY SITUATIONS
Consider that an aircraft emergency exists...(snip, snip, snip) when...(more snipping)
b. There is unexpected loss of radar contact and radio communications with any IFR or VFR aircraft...
The thing is, if you're getting VFR service from me all I know about you is your callsign and type. On the other hand, filing a flight plan provides SAR with lots more info, not the least of which is number of persons on board and a phone number that somebody can call when you don't cancel the flight plan.
Consider the fact that if you suddenly disappear whilst getting VFR radar service from me, the info in your flight plan will still be of great value. The same questions are going to be asked... what color is the airframe? How many persons on board? How much fuel? Route of flight? Contact info? I can't see any of that on the radar.
In short, it's not redundant. Two different tools doing two different (although overlapping) things. You'd be silly not to avail yourself of both when possible.
Dave