Both are important choose one base on your interest.
Elective subject does not mean that on is better and other is not.
Elective means both are equally important but you have given choice to take one which you like most or will be interested making career into it.
From working point of view

Building services - you will be getting mostly office job or site work which may not make you tired.
Modern Surveying - surveying is not a easy Job need hardship and really can make you tired.
Importance of both thing - equally important
Why elective - because subjects are wast and the Todays scenario is Engineering student cant able to handle it and institute dont want to make shit on their names in market because already many claims Indian Engineers coming out of collage are only having degrees and they cant be employed because they lack in engineering and no one wants to hire them.
So best choice is go with subject which you like most.