IMHO Lake Como is much nicer than Milan but it will be quite rushed in just one day if you plan on taking public transport. If you have never been to Milan before then I would probably stay local and visit the Duomo (on a clear day you can see up to the Alps), the opera house and perhaps - if you can arrange an appointment - even The Last Supper.
I used to live on Lake Como and still feel like it is my favourite place in Italy! If you decide to go to Lake Como you can take the train to Varenna and cross from here to Bellagio by ferry. Its worth checking the timetable but from here you may be in time to catch a hydrofoil ferry back down the lake to Como and return to Milan from here (it takes you right past George Clooney's very lovely villa - the Villa Oleandra) which is close to Cernobbio.
Alternatively you could hire a car and drive along the lake to Menaggio and take the boat across to Bellagio from here - there is usually parking available at the Menaggio ferry station.
Have a great time whichever you decide
