They are similar sports, but with slightly different rules.
Like comparing beach volleyball vs regular volleyball. They are both still volleyball, but the rules and scoring are a little bit different.
If you are awesome at regular volleyball, you probably won't suck at beach volleyball.
The biggest difference you will notice between IDPA and IPSC is in the length of the shooting sequences.
IDPA stages are maximum 18 shots, while IPSC stages can be much longer.
There's a bunch of other smaller rules that are different, but that's the main difference that you will notice.
As for which one you should join; The answer to that is simply which sport is more active in your area. Some areas have a huge IPSC league, and very little IDPA going on. Other areas have tons of IDPA events, but very little IPSC.
Visit your local ranges and see what clubs are shooting where. That will bring you to your answer.