Translation:If you learn Swahili, you will be able to talk with Tanzanians
I can't answer much of your question, but they may simply be synonyms. I think "ikiwa" is possibly a bit more clear, since "kama" can also mean "like/similar to". Also, "ikiwa" is itself the conditional form of the verb "kuwa", with "i-" (class 9, singular N class) as the subject. It basically means "if it is" on its own.
I think I saw somewhere that there's some variation in the verb tense used in the "if" part of the sentence. I think they should explain it a bit better in the tips and notes section.
I am a non-native speaker, so FWIW, I feel more comfortable when a simple tense (future, past or present) follows "ikiwa": ikiwa atakuja, ikiwa anakuja, ikiwa amekuja, ikiwa alikuja. I'm not very comfortable with "ikiwa akija" -- that I would just shorten to "akija". Could be a regional thing, or a "native speakers aren't bothered about it" thing. I am fairly certain I once heard a native speaker say "Iwapo kama watapata bunduki .." (If they get guns ...)