Gens is an old and not very accurate emulator. Bizhawk uses the Genplus-GX Genesis emulation and is very accurate, most notably in the Audio department.

Any new Mega Drive / genesis submissions should be recorded in bizhawk and I may even deny any submissions I suspect are Gens recordings. The emulation is just so much better in Bizhawk and just as easy to use as Gens.

Just note that when rendering the Avi, you will need to enable the pad option and set a resolution of 320x224 otherwise the avi will split if the game changes between high and low resolutions.