'What is the more favourable route considering security and road conditions?'....
That’s a very broad question to answer, surely it depends on yourself and what you decide you want to see/do.
As you look more and more into your route, new factors will come into effect and there are plenty.
Geographical, meteorological, political, historical, personal, physical.

When I arrived in Tunisia I still wasn’t sure whether I would go south via Algeria or Libya, fortunately for me, I decided to go east.

'All I really want is get an idea as to how many months?'...........
I had to unfortunately get home to South Africa from Uganda in a very short time.
My route was as follows:
Tanzania, 5 days
Zambia,4
Botswana, 2
If you didn’t have any accidents, breakdowns, border, political or health problems, I would imagine that you could cross Ethiopia in 6 days, but you would have to be fit and your bikes in good condition and regularly maintained.
Sudan,4
Egypt , 2
IMHO, this is not really the correct way to approach Africa as you won’t be able to form any opinions as you pass through each country.

A good place to start your research is all the strings on this website, also go to the links page and have a look at the hundreds of websites from people who have already done what you are looking at doing.