The old Dual core vs Quad core debate.
It all depends on what you will use the cpu for.
EDIT: great info above..
I don't have anything optimized for multiple cores, so no idea how much more beneficial 4 cores will be over 2.
i7 720QM is a quad, but the clock speed with Turbo Mode goes like following.
1.60GHz Base
1.73GHz Quad Core Turbo
2.40GHz Dual Core Turbo
2.80GHz Single Core Turbo
i7 620M is like this:
2.66GHz Base
3.06GHz Dual Core Turbo
3.33GHz Single Core Turbo
See the clock speed differences? Even in heavily multi-threaded apps it'll be a wash because of the massive clock speed gap.
I'd go for the 620M, it'll be faster in most cases and competitive in the worst, and with better battery life and lower heat.