Contactors tend to be a more 'professional' product, better specified, easier to use in a system, more expensive. The system designer will tend to use contactors, the module designer will tend to use relays, for reasonable meanings of module and system.
Contactors are more likely to be specified to be properly off when off, maintaining adequate contact separation to permit working on downstream equipment, which relays will not be.
Relays can be put in parallel to improve their carrying current, if you dress the lead lengths properly.
Relays should never be expected to improve on their switching current if used in parallel, because of timing. One set of contacts will be making first, or breaking last, and it's this set that will see all the wear, and fail very early. If it fails short, you'll notice, but if it fails open, you won't, as another relay will take over on point. This is the so-called 'zip fastener' failure mode, as one by one, all the damage is concentrated in each relay in turn.