Ground coffee or beans I have just recently started drinking coffee, and have got myself a small counter top espresso machine. 

Some people in work have suggested that instead of buying ground coffee (I have been using Lavazza ground coffee) I should buy beans and grind it myself. I have asked why, but no one has been able to give a better answer than "it just is better", so I am hoping someone here can help me find a better answer to this question. Or is it just personal preferences?

Also, as a newcomer to this, I know that the coffee someone uses is down to personal tastes and what have you, but I am just wondering if there are any coffee manufacturers who are very good when it comes to espresso (I mostly drink espresso).
Ground beans have a much larger surface area by weight than whole beans. When the surface of the bean is exposed to air chemical reactions take place. I doubt that these reactions have been completely characterized but at a minimum they include some loss of aromatic oils. In other words, the processes that lead to a judgment that coffee is "old" happen faster in ground coffee.  

You can do the experiment easily and cheaply by comparing the results of coffee made with freshly-ground and pre-ground versions of same brand/date coffee. You can always borrow a grinder.

The experiment might not work if both samples of coffee are old (or poor quality). Otherwise I think you would notice a difference. Whether the difference is worth paying for is a different question; but the only costs are a grinder and time to grind the beans. 

A short answer to the question about brands might be: good, fresh Arabica beans are similar in many ways, up to roasting. Some people prefer a light roast, others medium or dark. Once you decide, find a brand that does what you like consistently.