RTBA,
In QuickLOAD the RL15 actually does better with the 120 grain Sierra SP because the RL22 is already compressed 5% before RL22 gets out of the starting load range. 5% is the default compressing limit (you can alter it if you go to extra trouble). But the RL15, being more compact, let me reach a higher pressure and was able to get another 68 fps over the 105% RL22 load as a result. I have to say that while QuickLOAD matches Alliant's velocity very closely, it winds up with a charge that exceeds the Alliant maximum by several percent. So, YMMV.
With the Sierra 130 grain GK the maximum velocity from 105% compressed RL 22 and from a lighter load of RL 15 were very close, with the RL 22 edging out the RL 15 by 10 fps. However, it too 18% more RL22 by weight, to get that compressed load to do that. Hardly worth the powder cost. So, at least with these two Sierra bullets, if you can't buy more powder right now, then your RL 15 is where I would go with it. The wasted and unburned and more massive charge of RL22 will increase recoil about 12% and increase muzzle blast with no useful return on investment unless its barrel time just turns out to be better suited to your gun's accuracy nodes.