It really depends on what your team is like. If you're battling in UU with Flygon then it makes a great revenge Scarfer. Capable of outspeeding Heracross and many other pokes it makes a good scarfer. But if you have another poke on your team already running Scarf then Band Flygon can pack quite the punch. Golden rule of UU "always have a Pokemon that can outspeed ScarfCross" (A rule I often break)
As for Manectric. He's got a great Speed stat and with the nature you're running he would safely be able to run Specs. The average base speed for RU (Manectric's tier) is around 90. This means that with a Timid nature and max speed he would be capable of outspeeding most of the non-scarfed tier. With a Scarf however he functions as a great pivot and is able to build momentum quickly. Again I would base it off the fact whether or not you already have a Scarfer or Specs in your team, but Manectric can easily be both.
Hope this helps you in choosing which Item benefits your team more. If you want me to figure it out for you just comment with your team and their items and i'll give you a more detailed decision.