Based purely on overdrive tone, I prefer the Green Overdrive. It gives you a wider range of tones with the extra toggle switch.
The SP2 is a great pedal, though, and it adds that boost channel. So you can get two separate gain stages (sorta), or you can have a solo boost. It's a great sounding drive too, but the extra versatility is really the reason for getting it. Unless you have no use for a boost channel, of course.
Then of course there's the matter of pedalboard real estate.
If I were building a board where I had everything but the overdrive sorted, it would be the Green Pony. If I were wanting to keep things small and maximize space and power drops while still needing OD and boost, it would be the Silver Pony.
Hope that makes things even muddier for you!