There is no simple answer to this question, and it should not be treated lightly. Is the aircraft yours, do you have an intimate knowledge of it. If not, then it realy is not your problem. How close do you think you will come to the time limit? will your planned trip come in within the hours left. Your aircraft will not necessarily fall out of the sky at 100 Hrs if you do not do the maintenance, however, if you do the maintenance it does not necessarily mean that it wont fall out of the sky at, say,1 Hr into the maint period. Infact IMHO the most likely time to have a failure is immediately after maintenance. ( I can site many examples of this happening).
At the end of the day it is up to you, how comfortable do you feel? Remember, A 100Hrly is a minor service and wont necessarily protect you from disaster. There is no doubt, however, in the event of disaster, if the planned time is over what your MR has left, then your insurance will not cover you. REMEMBER only flight time counts.
PS. 15 Hrs is not peanuts if you are a private owner (IMHO)