Should we assume that the iMac and the laptop will be used roughly equally? If so, most people in this situation seem to prefer keeping the catalogue on an external hard drive (I think Peter Krogh called this the colostomy bag method!). It's then a matter of access speed.
One other issue is your presets and templates which would normally be local to each machine. You may want to check the "keep presets with catalog" option - it causes confusion in other cases
, but works OK with catalogues which live on external hard drives.
A possibility is to put the catalogue on something like Dropbox if your upload speed is sufficient. It can work OK, but you have to be very careful to check that the catalogue has been correctly synchronized before you open it on the other machine - otherwise you can easily overwrite work. OK, Dropbox keeps an infinite number of backups, but I still think this is risky.
My own preference is for the File > Export as Catalog, Import from Another Catalog route. But my needs are more than I occasionally need to take sets of pictures onto the laptop rather than continually switching.