Having just come back from a 6-day trek in Ethiopia (Simiens including Ras Dashen summit), I can definitely say that you won't be disappointed if you go. If you have more than a week you could also visit some of the tribes in the valley as well as the Danakir Depression).

I don't know anything about Togo/Ghana, so unfortunately can't offer a comparison. But Ethiopia was very affordable, I felt safe and never felt harassed as travelers sometimes do (e.g. in hotspots like Tanzania/Kenya or India).

I forgot to mention... you should the conditions during the month that you go. Scenery and trekking conditions will be different and I don't know how far into the dry season it is now

I've been to Ethiopia, Ghana and Togo. All were great. One thing with Ethiopia is its mostly very high altitude so the climate is cool and pleasant and in most areas you do not have to worry about malaria if you stay above 5000ft (1500m) there are no mosquitos. Ghana and Togo and I also went to Benin I would really recommend you add that to your itinerary if you choose that part of Africa, do less Ghana and more Togo and Benin, the northern parts. The area is mostly flat savannah so definitely go in the dry season, its dusty, so don't pack anything white! There is a malaria risk in Ghana, Togo and Benin so you could bring anti-malaria pills. The culture is great, the people very friendly, you can visit some of the villages, meet the local tribal kings, etc. There are also game parks, Pendjari in northern Benin is great, it takes some getting to. I can give you more particulars and exactly where to go and how to get there once you decide which trip you plan to do. But I hope this helps your decision process.

thanks guys for your experiences!
I had forgotten about malaria, so this must be considered, too...
Benin and Togo are my primary 'targets' of that area, but flights work better-cheaper for Accra, so, in a month's time maybe should confine to Ghana-Togo and leave Benin for another time.
Ethiopia could be explored easier for the same period, although I read that travel by public transport is slow, difficult and for Omo valley and other remote places, most people go on packages, which adds complications and expenses...
have to read a lot more...

Togo and Benin are slivers of countries in size, you really can do both its not far. Yes there's the malaria issue but bring the pills, no sweat...literally The more interesting parts of Togo and Benin are in the north and public transport would be very slow pretty much bush taxis and hope they don't break down. The main north-south road in Togo I have never seen so many broken down vehicles in my life, constant driving around down vehicles, much longer drive than you've think, don't count click and assume its 3 hrs, try 6. We weren't on a shoestring and had rented a 4x4, took on 15gallon spare fuel just in case but otherwise no fuel issues but there can be, you just don't want to find that out the hard way. You can do the coast, see Lome, Contonou, etc with more services but its not like in the north really seeing the place. Good luck if I can help any more let me know.

More on Ethiopia, you can do a local flight for Omo or you can overland via Arba Minch which is what we did. We hired a private 4x4, not self drive, we have a driver and good thing, nothing is marked and he even had trouble with communicating, all the tribes have different languages. But he certainly did better than we would have. I don't know about budget type travel there. Our 4x4 with driver was $150/day plus fuel, but that included his lodging and meals. If you go north you may not need as much travel support. A local flight or bus may do it. Very interesting things north as well.

many thanks for all these details!
The cost you mention $150/day plus fuel, was per car I guess, no matter how many passengers on board (or is it per passenger?)

I think the hydroelectric dams being built will change the villages and people along the Omo River. If you are interested in the people of the Omo Valley I would go there now and go to Ghana-Togo another time. I went in Sept 2011 and it was an amazing trip. I can only comment on the Omo Valley because I didn't have more time to see other parts of Ethiopia. http://www.pbase.com/cjw/2011_tribes_of_omo_valley_ethiopia_some_natural_nudity

these were great photos!!!
did you have any trouble with the people to get their photo, maybe they ask travelers to pay them?