"Let us know when you arrive home!"
Translation:Eve ulaştığında bize haber ver!
It really is hard to think of a difference in meaning now. All I can advise is that -ınca is always impersonal whereas -dik is not: "İstanbul'a varınca seni ararım" versus "İstanbul'a vardığımda seni ararım". You will get the ever so subtle difference in time; until then, just use either.