Did the night tour a few years back; went over on the first boat when it was still bright and got to see the island for about an hour before the sun set. So if you don't know whether you would prefer to see Alcatraz in day or night that is the best way to do it. Personally I thought the night part was much better. Also on the night part they do talks and stuff which they wouldn't do during the normal daytime trips.
Also to reiterate what other posts stated, you have to book at least a week in advance if you want any hope of getting to Alcatraz. I ddin't know this and was going to get tickets when I as over there; a guy at work told me about it two weeks before I travelled,thankfully!!!!!!