+1 to @dlcockrum 's post.
I have had both in my system and the difference between the two was more significant than with past SR upgrades (Note: this was with an SRX cable to the PowerCell Galileo SX, which likely played a significant role...as dlcockrum points out).
My advice is to go as high in the line-up your budget allows for. SR's trade-up program takes some of the sting out of future upgrades.