Both are really great phones, so it really comes down to preference. Do you prefer a larger screen or the slightly more portable screen? I myself actually prefer the 4.3" screen because my previous phone was the Infuse 4G, which had the same screen as the Skyrocket. That was a bit too large for me, but for some, that extra 0.2" is a really big benefit. So I would first recommend playing around with both screen sizes to see which one feels better for you.
Both phones are more than capable at processing. Yes, the Exynos on the original is absolutely beaten in its rave reviews, but really, unless you moved from using the SII to the Skyrocket, you probably wouldn't ever notice the difference.
Screen color is a bit different as well. I don't know why, but the SII has an even more saturated screen than the Skyrocket or the Infuse, which are both very saturated to begin with. I don't know if the higher pixel density attributes to that or not, but I actually prefer the colors on the Infuse and Skyrocket because they're less dark.
But the bottom line is, both the SII and the SII Skyrocket will not disappoint in every day use. Really, it's preference. Also, since it'll be your baby for two years, I'd also look at the form factor too. I think the original SII looks better, but the Skyrocket comes in white.