You will definately have to take what I say with a grain of salt. I have cast almost everything in the Sage line with the exception of the larger weights (and the spey lengths)
. In the lighter weight rods the action between the two is night and day. In the lighter weights the XP has a tip action that lends itself to tight loops and accurate casts. It is still a powerful rod by industry standards. The RPLxi is a progressive action rod. The blank will bend deeply for long, smooth, powerful casts. Like Juro said they are really two different rods with two different actions, for two different missions. Personally I wish they had made the SP+ in a 12 wt. I do think it probably says something that they only make the XP up to a 10wt though. Maybe its just marketing, or maybe the fishing conditions of the larger rods require a progressive action of an RPL blank. Although, I don't think anyone casting a 14 wt at a billfish is really worried about how tight a loop they make. They probably are looking for a rod they can fight the fish with.