If depends on if you're getting a discrete GPU or not. With a discrete card, the i5 460M is the way to go; without, the HD3000 on the i3 should be good enough to "even out" the CPU performance deficit.
The i5-460M is actually a better processor, especially with turbo. SB isn't much of an upgrade compared to Arrandale/Wastmere in terms of CPU other than aggressive Turbo, which the i3 lacks. However, the IGP difference is like night and day, with HD3000 you actually get a semi-usable GPU if you want to game, plus you have Quicksync support, while the original Intel HD was just miserable.