The scenery in the north is more scenic, but also more mountainous and therefore more of a slog on a bike. You can cross the border and take the Ekok-Mamfe road all the way across or take the road that head straight south to Limbe (the road that passes close to mount Cameroon). Across to Bamende is the more hilly ride and then south to Yaounde from there.
Biggest question is whether it is wet season still when you get there. When dry, all those dirt roads are fine and you will hear varying reports of how hard or how easy the roads are and this mainly depends on whether they crossed when it had been raining or not. At the beginning of June, the rains had started at the roads in question had become a very tough ride on a motorcycle, i.e. riding on pig snot. Probably passable on a pedal bike riding very carefully, I did see some locals pedalling gingerly around and with the light weight of a pedal bike and good tires much easier to handle than a big bike. If the weather reports of that specific area are still very wet and you become worried, you can always take the overnight ferry from Calabar to Limbe and that circumvents the mud roads and puts you back on tarmac.
Gabon is wonderful, main question for a pedal biker is whether to cross the border on the east (east of Franceville) or in the south. The east has a couple of hundred kms of soft road including sand on the DRC side of the border, so I'm guessing that almost all pedalers cross in the south.
Sorry about the brief answer but if you have any questions, please feel free to pm me and I'd be happy to elaborate. Safe ride!