Test ride them: the "best" bike is the one you're most comfortable on if you are intending to ride long distance. Other than that, your budget will be the biggest factor.
Dropped bars will give you a variety of hand positions: over distance your hands and wrists will get tired and sore stuck in one position (as with straight bars).
You will also be able to adopt a lower, more aerodynamic position. I forget the exact numbers but because of wind resistance, to go (for example) twice as fast requires something like four times the power. This will make a big difference over 300 miles.
Many long distance riders still ride steel framed bikes, because steel is springy and absorbs vibration. However, most modern bikes are made from either aluminium or carbon fibre: aluminium is considered more rigid and a harsher ride, but cheaper than carbon fibre. An aluminium frame with carbon fibre forks can be an economical compromise.
Most of the benefits and disadvantages of the different materials are down to the skill of the frame builder: both good and bad examples can be found in all of the materials.
I would think that you will get a bit more for your money with a Specialized: it's not that Bianchi aren't good bikes (they are), it's just that there is a premium attached to the name like with Apple or Sony.
The other choice you will have to make is between a "double", "compact" or "triple"
chainset (second paragraph). Running and cycling use different muscles so although I assume you are quite fit, as an inexperienced cyclist and bearing in mind that you intend to ride long distance, I would lean towards a triple or compact.
I recommend
this book for a wealth of useful information and training programmes, all written in an easily comprehensible and not overly technical style. If you can get a copy before buying your bike, you might find the decision a little easier.
