That route sounds good and is keeping the visa situation simple. Culturally central asia is very different from Russia, they have a rich Islamic history and all follow the 'Turkish model' - they are an amazingly warm and friendly people. The old silk road cities of Buchara and Samarkand have some fantastic old buildings, the tomb of Tamerlane etc if you like that sort of thing. I went down through the Balkans, Greece > Turkey > Georgia . Azerbaijan > Ferry to Turkmenistan > Uzbekistan > Kazakhstan > Russia then headed back West. I was supposed to do TJK and go over the Pamir highway, having seen Colebatchs photos I REALLY missed out so have to go back next year and rectify that! If I was doing it again I'd take the southern route partly for this and also to see if I could ride the Nagorno-Karabakh trail in Azerbaijan on a bike. Its a walking trail, but it's in Azerbaijan and bears witness to what was by all accounts a very brutal and under-reported war between AZ and Armenia.
A lot depends on what sort of riding you want to do. If you want to do more off-road you need much more detail in the planning as to what is good and ultimately that will decide. The southern route would probably add 400 quid in beauraucratic bull, but may be worth it. If you leave in May/June either route will be fine weather-wise. Russia is amazing but the scenery can be a bit monotonous depending on how much you like pine trees!