The software should tell you or give you that option if it doesn't it should mention that to you. If you want to use keep 1:1 or close to it you should just get CloneCD as it will just do dvd5 to dvd5 or dvd9 to dvd9 format that would eliminate you having to figure is it compressed or not?? Only way to know without the software telling you is to do a property check of the original dvd property and the saved iso property and they should closely match cause that should tell you if it will fit on 4.7G or 8.4G media. Also if you put a 4.7G and it doesn't fit and tells you the media is insufficient then you will know it requires 8.4G media unless it is less then 4.7G then it will fit on a 4.7G media. But if the software is reliable it will tell you.
Dual layer is on one side of the dvd media and it is 8.4G capacity unformatted. To have on both sides is called double sided layer that is only available to my knowledge to the movie industry not to the customers. Those are called DVD10 media, I think.
That is probably related to the extra info each software writes so that their program knows size, format and properties needed to burn the info back onto the blank dvd. There shouldn't be any problem using one software to burn another software created ISO but my recommendation is stay with one software from beginning to end espically if it can copy and burn the iso back onto dvd media. I for one would say stay with CloneDVD and come with many users satisfaction. Even I have have it besides CloneCD and AnyDVD HD. Just remember that if your dvd is CSS protected you will need AnyDVD HD or not to decrypt it so CloneDVD will work properly on CSS protected media so that you can use CloneDVD to make your ISO.