If all the technical / logistical / environmental factors add up, I favour the Canterbury Crescent / Popes Rd scheme over the Effra Rd one. It could easily be added to if an extension to Streatham were ever planned.
Reasons:
The stretch of High St from the junction with Atlantic Rd to Effra Rd is already more than congested with both vehicles and pedestrians
The removal of the central reservation (planned to give more pavement room) will result in more people dashing across the road. Trams stop in the middle of the road (afaik) - people will rush past a stationary tram into the path of a moving car - loads more accidents.
Trams ('streecars') are used very effectively in Central Toronto, BUT Canadian drivers have been taught to drive understanding that you never 'undertake' a stationary tram for fear of flattening an alighting passenger. British drivers do not have this culture - and many who drive in S London cannot be relied on to drive with any patience, safety or respect - so a major tram stop in several lanes of traffic could be a disaster. Much better in the backwater of Popes Rd.
The Popes Rd stop would put the trams into the heart of the planned new interchange between the back of the tube station and the overground - the tram will be very disabled accessible - and with the tube now accessible, it will make Brixton a better transport destination for wheelchair users etc.