Is a 255 Gram Silicone Implant More or Less Volume Than a 255cc Saline?

CC's and gm's or ml's or all the same.  A 255 cc implant is the same whether it is a saline or a silicone gel implant.

If you are trying to get an idea about your size, I would recommend measuring out rice in a bag and just going by the volume. Implants available in the US are all based on cc's, but we sometimes mis-speak and say grams. As mentioned before, a sizer will look bigger on you that in you, so it is really only giving you an idea. The rice test is good for figuring out if you want a 300cc implant or a 500cc implant, not for telling the difference between 300cc and 325cc! The profile of the implant also has a lot to do with you appearance after surgery and that should be discussed with your surgeon. Good luck!

Since implants come in different styles, you can have impants with different width and projection having the same volume. Many doctors are using the dimensional approach to implant selection as do I. Compare the implants using the style of implant and dimensions rather than just pure absolute volume.

Although your question sounds very simple, it is, instead, very complex. First, none of the now standard implants approved by the FDA are measured in grams. That being said, there are several other problems with your question.
First, although you can get a general idea of what size you will be by using sizers externally, there is actually no way to guarantee that you will be even close to that size after surgery using the same implant size. Several things go into the apparent size you are after surgery. One is whether the implant is over or under the muscle. One under the muscle generally has to be slightly larger to appear the same size as one over the muscle.
Secondly, your specific chest and breast size and width will change the appearance of whatever implant is used. Dimensional Planning uses your size to select the base diameter of the implant. Once this is done, the projection can be selected. With the same base size, the different projections have different sizes.
The short answer, then, is to use cc’s to select about what you would like to be, and then discuss this and any other wishes thoroughly with your Plastic Surgeon so he oe she can take all these things into consideration to select the implant that will best meet your desires.

I am not aware of the use of grams in implant sizing for many years. The old Replicon polyurethane coated silicone implants used to be sized in grams as I recall but these have been off the market since around 1989. If you are having gels used, then have gels used as sizers in a garment to approximate the volume and look you want. I also rely primarily on dimensional measurements and profile preferences to determine what implants I want to use but find that the visual aid of an implant in a garment is very helpful for the patient. Just remember that the implant will look bigger ON you than it will IN you so this needs to be taken into account in sizing in a garment.

I am not familiar with silicone measuremnts being given in grams.  In my experience all sizes of implants are given in cc's.  Trying them on does give you some ides as to the look you will achieve, but only in a very basic way.  All the best.