I do not know anything about Allegro. Inexpensive road bikes are heavier than expensive road bikes. There also is some difference in quality between the very low end and the mid-level, but not between mid-level and high end. From mid-level up, you are really paying a lot more for smaller and smaller differences. Most of the difference is weight, not functionality. The higher you go in price, the more you pay for very small savings in weight.
The progression of Shimano components as you go up in price (from low to high) is Sora, Tiagra, 105, Ultegra, Dura Ace. Up to the 105 level, you are getting better shifting, easier adjustments, and longer life. Above the 105 level you are paying for lighter weight and better looks, but not noticably better shifting or longer life.
The progression of frame material is steel (inexpensive, heavy, long lasting - as long as you protect it from rust - and good ride characterists, but increasingly difficult to find), aluminum (light; inexpensive; stiff; theoretically more prone to metal fatigue than steel, but this is unlikely to be a real factor under normal use; rust and corrosion resistant; but does not dampen high frequency vibration like steel, carbon fiber, or titanium does), Aluminum with carbon fiber seat stays and fork (all of the good qualities of aluminum with some dampening of high frequency vibration in the places that matter most, but whether this is a real or theoretical advantage is debatable), carbon fiber (light weight, rust and corrosion proof, good ride characterists, but expensive and susceptable to hidden damage that could lead to frame failure without warning; however, like the metal fatigue possibility of aluminum frames, unlikely to be a problem under normal use - just have the frame inspected if you have a crash), titanium (combines the best quailties of steel and aluminum, but extremely expensive).
The advantage of buying a new bike (assuming you buy from a good LBS) is that you get a bike that is fit to you. You will pay more for what you get, but a lesser bike that fits well is far better than a better bike that fits poorly.
The advantage of buying a used bike is that you can get better components and a better frame for less money. The disadvantage, though, is that unless you know exactly how a bike should fit, you may end up getting a bike that is not a good fit for you. Some fit problems are correctable, but you will spend additional money to make these corrections. In the extreme case, i.e., the frame is just too small or too large to be made to fit yo properly, you get an ill fitting bike or, possibly, an unstable bike because your weight is in the wrong position. If you are lucky enough to get a good fit on a used bike, then you will come out ahead, but as I said before, it is better to have a good fitting entry-level bike than a poorly fitting mid to high end bike.