The Plant Factory graph system is most complex. TPF nodes are not only used to define plant components but also for L-Systems like control (ex. Loop node), conditional branching, randomnes (random input, random output) or to influence parametes inside nodes. Coloured 3D preview of model helps identifying various sections of the plant defined by distinct nodes. But TPF models with implemented age/health/season variability can use huge graphs convoluted by several orders of magnitude.
Speed Tree graphs are simple they look very nice. Only plant component nodes are available, no additional control nodes, parameters extraction, looping , neither plant colouring - only highlight of parts generated by selected node.
Xfrog graph system is a little bit more complex than one from Speed Tree. But lack of randomness in Xfrog has to be compensated by addition of additional graph parts to the graph, so sometime you may end up with pretty complex graphs even for very simple plants.
If quantity of nodes types would define the quality of plant editor, TPF would obliterate any competition with some 30 dedicated plant edition nodes + about 250 general purpose nodes. In reality, most of plants can be build just with single node type : branch (segment). Additional nodes can enhance control, precision, detail, realism, but Speed Tree with its 2 nodes does almost as well as TPF in majority of common situations.