In both of them you can draw your substrate (or products, or both) to find the publications/patents with the similar synthesis. Unfortunately, it works only for compounds described already in the literature.
However, you can draw only the part of your compound, which you suspect will react with chosen reactant/in chosen conditions, and find reactions that will be similar to yours.
I haven't used any software that can simulate complex reaction.
Using chemical databases such as SciFinder and Reaxys can help you to navagiate to millions of chemical conversations. If you draw reactants, you can know the possible formed products. For totally new reaction, you can focus on the functional group tran formation keeping in mind very similar chemical environment. Last but not least, if you already target a new structure, you can apply the retro synthetic approach to find the starting synthons to design your experiment.