For very brief context i've held the opinion that the former had it worse then the latter by a small margin and some friends and I were debating the idea im curious what someone far more educated's opinion is.
Not to forestall further comprehensive answers to this question, but There's some very good answers to questions similar to this in our FAQ on American slavery, including:
/u/sowser on whether there were Irish slaves in North America (and really, on what slavery actually is), including several links to other answers that are very much worth reading (and also in much detail here)
/u/sunagainstgold on 'No Irish Need Apply' should give you a sense of the very real discrimination that did exist against Irish immigrants, which nonetheless did not compare to the total dehumanisation of slavery.