Both will dye beautifully. Prewashing the fabric has the side benefit of making the silk dress made from it washable, unless the trim does not allow this.

You can dye silk with the acid dyes that can be used for wool, OR with the same fiber reactive dyes we use for cotton. Acid dyes will help to retain crispness and shine, while fiber reactive dyes will make the fabric feel softer and less stiff. Which do you prefer?

Lining? Oh, dear. I thought you were writing about unsewn silk, to be dyed and then sewn into a dress.

Lined garments are just about never washable, and silk garments whose silk was not prewashed before sewing are often not washable. I have never heard of an off-the-rack lined silk dress that was not marked "dry clean only." And of course, truly dry-clean-only garments are NEVER dyeable. All forms of dyeing require washing.

The way to make a silk dress washable is to prewash the material before sewing it into the dress. A silk dress, especially a lined one, should be assumed to have missed this incredibly simple and easy step, and to be dry-clean-only. When you wash a lined garment that was marked "dry clean only", either the outside layer of the garment will shrink, or the lining will, but they will never match each other. The way the lining fits under the outer layer will almost inevitably be completely ruined.

In contrast, if you prewash and dye some nice silk chiffon, then have it sewn together (there are thousands of tailors who can do the sewing for you!), the dress is washable. If the fabric, or its trim, is too fragile for machine washing, it should still be hand-washable.

There are many advantages to clothing that is washable, besides the obvious one of saving time and money by not having to haunt the dry cleaners. Dry-clean-only garments are susceptible to damage from water spots, which are so common at parties where food and drink are served. Water spots on a water-washable dress can be easily rinsed out. Furthermore, the solvent used in dry-cleaning is a probable carcinogen, so it is important to avoid extended exposure to it. Storing freshly dry-cleaned garments in a closet that is attached to a bedroom produces extended exposures; they should be aired out in another room, or, preferably, outside, to reduce this exposure. Of course, perchloroethylene is also damaging to the environment, and may have significant risks for those people who work at the dry cleaner's establishment.

My own wedding dress was sewn of a beautiful silk charmeuse that was prewashed in the washing machine before sewing. Silk just cut from the bolt may be prewashed and then dyed in the washing machine (see "How can I dye clothing or fabric in the washing machine?").