Since I have multiples of both certs I will throw in a comment here. You only need to take each core test once, after that all you need to take is the elective to get another cert.
As the first ACS cert I would suggest IP Telephony because that is a good general purpose cert and will make it easier to get the simplex and complex IP telephony Product Authorization. This means Avaya recognizes you as capable of installing S8xxx servers.
One person asked if you do not pass the tests on the first try, is it different the next time. Being a person that did not pass on the first try, yes the test is different next time.
I took the beta test of the current IP Telephony ACS test, there were 400 questions and I had four hours to take it. The current test is 60-70 questions so they a big pool to draw questions from.