Stick with trains
When you get right down to it from an energy perspective, trains are always always more efficient than buses when you know where your population needs to be.
If your society has any comparison to modern societies, your cities are going to be segregated into 3 main areas; residential, commercial, and industrial. These distinctions are already fully understood and the need for transportation of people between these districts is also already fully understood.
The main reason why most public transport systems across the world are not efficient is because they adopt a 'star' cluster model, which implies that commerce, usually at the centre of a city model, is the most important sector in any given city design. The problem with this is that it doesn't account for two primary factors;
1) Growth of Scale, and
In any economic model, industry is what allows a society to grow and function. Commerce is merely a method of churn within a society that is capable of generating its own industry. What this means in practice is Ring Roads.
If you have people living all around the outer edges of your commercial centre, you also have industries relatively evenly distributed around that centre. Commerce, by definition, needs to be in the centre of a given industrial society so as to allow the even distribution, export and import of good across that society. Industry on the other hand is better placed in close proximity to the source of raw materials and labour for that industry. As a result, ring roads allow for the populace to reach the primary sources of employment around the centre of the society defined by the city in question.
What all this means in practice is that if you know where your people live AND where your people work, then putting in tracks and train infrastructure is far more cheaper than building roads AND ever increasing sizes of buses and other capital transport investments to get people from home to work and back.
Ultimately, most cities are reasonably predictable. That means that you know how many people are travelling between point A and B at any given time, which in turn means that you can plan for rail options to move those people and have a far more energy and financially efficient model of transport for goods and workers.
Buses (God bless their cotton socks) are really about filling gaps in change between distribution conduits. If you have an established transit route, trains will always be more efficient than 'road train' bus and cargo models. That said, in a society where (geographically) there is constant changes in residential and industrial sectors, buses are the way to go because they require less up front capital expenditure. Most large cities however do not follow that model, and if you want an example of how a large organised society can fail to implement efficient public transportation via rail, one need look no further than Los Angeles.
In my home town of Canberra, Australia, this debate is currently an active part of local politics. The anti-light rail lobby always point to cost, and yet the local government is already restricting parking spaces in the central regions to the point that parking is becoming a blood sport. They say that it's in order to promote choice, without really giving an alternative. This is despite Canberra's bus company (ACTION) heavily investing in articulated buses.
Bottom line is that as any city becomes more heavily congested, the answer is always more efficient alternatives to road congestion. Larger and larger buses are not the answer in such cases; what is needed is a transport conduit that bypasses roads completely and which our DNA is somehow programmed to give way to, regardless of how congested the roads may be.
The answer to that is trains.