almost all external dvd drives found now are the thin laptop style which means that they will tend to be more fragile and wear out just a bit quicker. the only notable differences are typically what kind of enclosure they are in (metal or plastic) as well as styling.
apart from some various read/write speed differences which typically are not very far apart or any blueray playing capabilities (you didnt say you needed this) they will perform about the same as noted by the poster above. i would however try to use a well known brand if possible and do not think it is worthwhile to spend a big amount of money.
personally if i wanted an external which was more rugged and well built i'd think about using one of those 5.25 bay device...