This is very much a matter of taste and fashion. It is also determined by the bride’s dress. Your bridesmaids’ dresses really must compliment the bride’s dress. Contrasting styles can look disastrous.
Depending on the formality of the occasion you do not have to insist on all bridesmaid dresses being exactly the same style but they should ‘match’. Again, it’s very much a matter of how you want your wedding. Strapless dresses are very popular now as are spaghetti straps. Lengths vary but be sure to go for one length for all to look co-ordinated.
You might also want to consider that today’s bridesmaids wear dresses that can be re-used as evening wear, cocktail dresses, suits and so on. So when choosing, bear in mind that whatever the expense and whoever pays it is a nice thought that your girls will be able to wear their outfit many times in the future.