haha, I don't even know.
I love them both equally. TH3 had peak mechanics, but 2 more nostalgia for me. If I had to choose, THPS3 because I liked the gameplay and levels more. And the Xbox version in particular because it had extra stuff. Really close is THPS2 on Dreamcast.
One of my better refund actions was returning THPS4, which I didn't like much, and taking a cheap THPS3 copy + the rest of my cash back. I initially decided to buy 4 because I had played 3 loads on PC (which wasn't that good of a version), but I ended up playing the Xbox version of 3 for months.