See here's the deal
Your year of passing would be of the first marksheet as improvement exam doesn't mean u will pass or fail it's just that u improve your existing marks on passed subjects so year of passing is that of your first marksheet.
Now comes the tricky part roll number, it's up to you which one you want I'll suggest the improvement marksheet if it contains all the subjects . Usually it contains those in which you have to appeared and have scored higher in that case go with the one that shows all your subject marks. While entering marks change those in which your have improved and submit both Xerox . Or u can choose the first one as it will easily be accepted without anyone doing any fuss cause during admission a lot clerks don't know what they are doing so I suggest you go with your first marksheet & it's roll number unless your second one has all 5/6 subject marks printed on them
Note: year of passing is of first marksheet.