same meaning but "soba" is a noun and "chikai" is a adjective. and almost all japanease adjective can be noun. then, "chikai" -> "chikaku". kare no soba ni iru kare no chikai tokoro ni iru kare no chikaku ni iru same meaning. in addition, soba can use only for physical position and place but "chikai" can use for others. anata no iken(opinion) wa kare no iken ni chikai (cannot use "soba") anata no nenrei(age) wa watashi to chikai (cannot use "soba")