Straight from the horse's mouth. I hope this spinoff talk begins to vanish now 
"Of particular note is that Kojima refers to both games as "Metal Gear Solid 5." He describes Rising as a "next generation MGS5." He describes Peace Walker as "The MGS5 Inside me." "
The article also mentions his role in the production of both games:
"The Metal Gear series father is serving as director, producer and game designer on the PSP title.
As for Metal Gear Solid Rising (confirmed in Famitsu to be for Xbox 360 and PS3), Kojima is serving as producer along side Kenichiro Imaizumi. He told the magazine, "I'm not the director. I'm not doing game design either. Although, I do issue complaints (laughs). But, not too much."
http://psp.ign.com/articles/990/990460p1.html
