The FSM has just about everything - BUT the ALLDATA has all GM TSB's and some (at the time) parts prices plus the standard and warranty labor book times, plus descriptions of system operation, diagnostic paragraphs (including the GM trees) - so it depends on whether you want a laptop in the garage or a paper set of manuals.
I'll admit that I'm partial to ALLDATA because it allows me to give the 2000+ owners labor rates and diagrams as saved downloads.
I have FSM and I use it a lot, and also I have used AllData in the DIY center that I regularly go.
I feel much more comfortable with my FSM, since everything I need for a job is collected near to each other. But in AllData I should browse back and forth to find different information that I need for a same job. However AllData has more information than FSM.