They will probably resemble their parent. Some lavender is sold as seed strains, even though you would expect all 'Hidcote,' or 'Munstead' say, to be vegetatively propogated. So you would generally get darker-colored flowers from 'Hidcote' seeds, lighter-colored ones from 'Munstead,' but with the occasional oddity -- a really fragrant 'Hidcote' would be a nice surprise.

If you grow several types of lavender, chances are better that some crossing takes place and the seeds will vary from the parents, but that is a good way of getting plants that are out of the ordinary -- longer stems, interesting arrangement of blooms on the stems, interesting color, special fragrance, etc.