if you plan to use only floating lines, or short tips lines, then 15' rods in 7 - 9 wts are often nice and easy over a long day
if you plan to use med/long line tips, then a rod with a 9 in the designation is a minimum, and a 10 wt gives noticeably more authority. These can also be used for floating line work very effectively, but carry the curse of added heft if you aren't going to take advantage of the power
the Sage 9150 rods, regardless of color, are the same stick. The Sage 8150 is the older and softer version
there are lots of 15' rods on sale in classifieds here and elsewhere, as people move towards short rods and shooting heads. Recommending any particular rod depends on your preferences with respect to action and applications (which you haven't stated), just as it would be with recommending a shorter rod.
CF Burkheimer fly rods advisory staff
Nextcast lines pro staff