The data processing is sequential in server jobs.Server jobs need lesser startup time as compared to Parallel jobs and perform well for small amounts of data.Most of the transformations need to be done in the transformer stage.
The parallel jobs can process large amounts of data in lesser time when compared with server jobs bc of the parallel processing capabilities and the start up time is more as Datastage has to devide the available resources(prcessors) as nodes for attaining the parallelism.
There are many parameters like volume of the data to be processed, number of physical processors available on the server, buffer sizes,..etc to decide which type of jobs are better(server/parallel).
Generally PX jobs run with more throughput but take more time to develop. Let's take an example job that takes 3 days to develop in PX and runs in 10 minutes and the same job would take 1 days to develop in Server but runs for 60 minutes.