For many riders, starting early in the 90 hour Group Start means getting to Carhaix (525 Km) to sleep on Monday evening. That is a long way to go without a long stop. The later Free Start makes Loudeac (440 Km) a more natural stop point. It also makes it possible to stop for the night in Loudeac on the way back, and if you have a hotel room it may be easier to book it for two nights in a row.
One the other hand, starting in a small group and perhaps having to stop for traffic (no motorcycle escort for the free starters, I heard), may slow you down. And if it doesn't, you will catch the tail end of the group start riders, and be delayed at the controles anyway. You also miss riding in daylight for the first 2 or 3 hours.
My suggestion is to pick the group start, and if you decide to start late, then start with the last wave, which may leave shortly before the Free Start. In general, unless you have a good reason to choose the free start, I would not recommend it.