The number one target for any LD bike is comfort. Whether than comes from a frame made of steel or CF or aluminium or titanium is, to a large extent, decided by your preferences, tolerance to discomfort and finally experimentation.
Weight can come into the equation, simply because an extra three or four pounds will require extra energy to push along the flats or up the hills. However, to take full advantage of that lighter weight in the bike, you will have had to have pared down the weight of the stuff you already carry on randonnees, as well as your own bodyweight to a level that enables you to have enough in reserve to cope with deficiencies in energy intake.
In some cases, saving weight on a bike also means trading off reliability. For example, lightweight steel frames might have a weakness in the right-hand chainstay. Or a crank might break. In some cases, lightweight cassettes have been bent. You have to weigh up the risks of these things happening in the middle of nowhere in the middle of the night, against your desire to qualify for, say, PBP2015.
And finally, the average speed you are going to achieve over a randonnee is dependent almost entirely on your own fitness level. It's cheaper and likely more productive to concentrate on improving your endurance and speed over long distances, than spend more on a new bike.
However, this is all in the context of the Eros already being a reasonably light bike (I don't know anything about the Bianchi range). You don't post what the comparative weights are between the two models. Remember, about 2 lbs of weight saved is the equivalent of a one-litre bottle of water. If you notice a speed increase when you ditch a litre of water off your bike, then you might find some benefit; but I doubt whether that will be the case.