According to the database on another website (link below) they appear to be one in the same.

http://www.shootgardening.co.uk/plant/helianthus-annuus-russian-giant

Thank you, Scottish! It's still a bit unclear, since the actual name 'Mammoth Grey Stripe' isn't on that list of names, but I wouldn't be surprised if they were the same. Funny that a single seed company (Botanical Interests) is selling both names, though. I've got a call in to them, but am still waiting for someone knowledgable to call me back. I wouldn't fuss any more, if I hadn't learned to be paranoid about the exact variety name.

Hope you get your answers TBH!

Well, I got a few answers back! The expert at the seed company called back, and her answer to my question was, "Very subtle." Apparently, there are a number of genetic lines that are sold under one name or another. The one that they sell as 'Mammoth Grey Stripe' is possibly more properly named 'California Grey Stripe', and tends to have slightly larger seeds, with more stripes, on a grayer background than the 'Mammoth Russian'. The disk flowers on Grey Stripe also tend to be a dark brown, while the Russian's tend to be ochre or tawny in color. Otherwise, similar sized flowers on similar sized plants, with similar needs.
Thank you for your help and interest, Scottish! : )

The only way to find then is to grow them and find out for sure!

Ahh, for an acre of land! Wait a minute...WHAT did you say the property taxes and water bill were?!?! ; )

Forgot for a minute where you were T....water rates will be sky high!!!!! Don't have that problem in Scotland, bl***y rain!!! ;)