A I might start by clarifying some terminology. The RJ45 plug is part of a networking standard which, as a whole, is called 10BaseT. BNC/coax is part of a standard known as 10Base2, which is how I will refer to it here, even though a lot of people just call it BNC. Both are examples of Ethernet networks.
When you have only two computers, you can use a single 10BaseT crossover cable to give you a length of 100 metres. This means that you don't need to use a hub. You should be able to buy a crossover cable fairly easily.
10Base2 cabling will give you slightly more length -- up to185 metres -- and you can have more than two computers connected to the same cable.
If you add a hub to your 10BaseT network, each computer attached to the hub can be up to 100 metres away. Given that that you can get hubs for under $100, and that 10BaseT cabling costs less than 10Base2, this can be the cheaper option for longer distances. However, the location of the hub is critical. Obviously you can't have it sitting on the footpath between your two buildings!
You might be tempted to try longer lengths of cable to see if they work. Don't succumb to temptation! When a computer wants to transmit on an Ethernet network, it first checks if the cable is busy, and, if it isn't, it transmits a message telling all the other computers not to transmit. Sometimes another computer might do this at exactly the same time. This is referred to as a collision. By detecting the collision and having the two computers wait for a random period of time before trying again, the cable can be shared. Failure to detect collisions properly will result in network drop-outs, often requiring computers to be restarted. If maximum cable lengths are exceeded, drop-outs are much more likely.
- Roy Chambers