Some considerations:
1) Formation flying requires considerable training and practice. Only do it if you are current.
2) Formations can fly in any flight regime, even Class A airspace in IMC. All communication is with the leader and only the leader squawks.
3) You are less likely to collide in tight formation than "same way same day" formation because you are more assured of remaining in visual contact.
4) Formation flying is tiring. Is it a good idea for a long trip?
5) Formation flying requires more throttle movement, meaning that adjusting power and mixture can be less exact and therefore less economical.
6) Formations always fly much slower than even the slowest aircraft, to allow for closing and outside turns. This will result in the long flight being longer.