Using an outboard DAC will improve the sound quality. If you use a DAC you have to use a headphone amp (built into the DAC or separate) since you are bypassing the one built into the iMac.
If your headphones are high impedance or difficult to drive a headphone amp alone will be an improvement but not a much as using a DAC too.
Most apple computers especially the pro oriented ones seem to have some pretty good dacs, so an amp is probably all you need, however most devices for sale will do both, and as for which one is a good option, well thats an audiophile black hole.
Are you certain you need an amp? unless you have some exotic headphones, chances are they will drive just fine.