If you're organizing a conference, you'll need more than a simple assembly hall or space with a lot of chairs. The hall should be located in a building with an attractive façade and plenty of parking - participants in your conference will only be with you for a day or two, and making a good first impression is vital. You'll most likely need equipment for projecting video and sound (and you may need simultaneous interpreting capabilities). You may also need a separate space for registering participants and coffee breaks.
A classic training session works best with an absence of tables, a loose configuration of chairs, and a flipchart. Training sessions often last for more than 3 hours, so you might want to make sure that beverages are available at the venue (in the form of a water cooler, or perhaps a kettle).