I like Morocco
I was lucky enough to have the opportunity to ride through western Africa a couple of years back—I rode from Nigeria to Ireland, along the western route through the Sahara, taking in Morocco on the way.
I heard a lot of 'horror-stories' about Morocco, mostly talking about the level of petty, and not-so-petty, theft in the country. I arrived in Morocco, from Mauritania, not looking forward to it all that much, and hoping to escape unscathed. I left wishing I had more time to spend there, and completely enchanted by the place! I'd go as far as to say it was the highlight of the trip. It's definitely on my list of places-to-be-revisited. I just found the place captivating. The people are friendly; there is beautiful scenery; the cities are mystical; the moutains are alluring. Morocco, for me, has a huge amount to offer. As Mike already pointed out, Morocco has three months' worth to explore—I concur!
It does sound like your proposed route is going to be be a tad pressurised if you're going to fit it into just three weeks. I remember our trip being a 'little bit tight' every now and then, and I swore, if I were ever doing an over-land trip again, I'd make sure I had enough time not to give a toss how long I decided to stay in any particular place in which I felt the need to linger.
Time is precious, but racing through a trip just to tick off all the countries/places visited is a false economy. Just my opinion.
Regardless, you'll have a great time in Africa. If it's your first trip there, you'll feel the excitement and anticipation all the more. Whatever way you decide to do it, you'll enjoy it immensely.
Good luck, and don't forget to report back to us how you got on!