First thing, check that your wheel rim will take the narrower tyre, it probably will but some hybrids come with wide tyres and rims and won't.
The advantage of a narrower tyre is supposed to be that it is quicker, but whether a 25 is going to be noticeable faster than 28, is hard to say. Also the narrower tyre the harder you have to pump it, this does tend to lead to a harsher ride, unless you are riding on a smooth surface all the time you are unlike to increase you average speed significantly...