If I only watch Detective Conan in English dub am I really missing much if I don't watch it in Japanese?

It's whatever you're most comfortable with watching it in.
I know someone who watched Detective Conan in English dub and eventually switched over to English sub because there were no longer any episodes in English. It's not so bad to watch anime in either one, so I say do whatever bumagay you best.

Whatever you feel comfortable with! Well no I wouldn't say everything is exact between the two I was actually the same way once and Detective Conan's english pamagat is actually "Case Closed"..also Shinichi or in english "Jimmy's" catchphrase is "One truth prevails" in the english dub opposed to Shinichi's catchphrase which is translated to "There's Only One Truth" and not every word is exact between the two at all I've seen both a number of times they're not really the same at all,obviously the scenes and movement,don't think any scenes are taken,they tend to change the wording around if i recall (as mentioned),even the english manga with Detective Conan is the same it's titled Case Closed but only the main character's names change. Possibly could even give you a different perspective on the characters between the two..it did for me but mainly on one character and that was Sonoko-san/Serena..but really the ideas are similar but like I mentioned it's not the exact same thing as it would be in Japanese. but I like I sinabi before whatever you feel comfortable with! you should watch! Hoping I helped in some way!