are the only differences between n and g speed and coverage distance?
if so, how much?
This is a very tricky question, as there isn't any one valid answer. "g" or "n" by themselves don't mean much as the devices vary a lot, as do the local circumstances as well as client devices.
For general rules of thumb, draft-n has significantly higher throughput
capability than standard-g, but this potential can be hampered by your local conditions to be not much more than standard-g, and certainly well below the marketing in practice. It depends. The claim is also that it gives better range, but the typically finding is not that it goes out much farther, but that it gives better throughput at the same distance verses standard-g. Of course, given any two devices, there will be differences in range, but there are undoubtedly some standard-g devices which give better range than some draft-n devices, especially when you consider standard-g devices with built-in wireless amplifiers such as the Buffalo WHR-HP-G54.
There are also of course differences between the antennae and radio design of various routers, and two different routers sharing the same radio and similar antennae designs could still have different wireless performance due to differences in software.
"54 Mb/s" standard-g marketing translates to around 23 Mb/s maximum actual throughput under ideal conditions. This means around 15 Mb/s in the normal non-ideal conditions.
"300 Mb/s" in draft-n marketing is bunk, especially when combined with only 100 Mb/s wired ports. This translates to somewhere around 100 Mb/s maximum actual throughput under ideal conditions, and more typically around 60 Mb/s under normal to good conditions, but also down to 30 Mb/s or less sometimes -- especially when standard-g devices are on the network or in range.
Under crowded wireless conditions, it's good to have 5 GHz capability, but 5 GHz has its own issues with range and obstructions -- it doesn't work as well as 2.4 GHz in these areas.
SmallNetBuilder has nice comparison charts for throughput vs. distance, which these give you some real-world throughput figures, but are only valid for comparison with other devices on the same site. I.e. it's a very specific test using the author's home or business, and you'd get different results in a different layout, and probably better best-case results with less challenging conditions.
http://www.smallnetbuilder.com/component/option,com_wireless/Itemid,200/