The i7 720QM is a quad core clocked at 1.6 GHz which will Turbo Boost up to 2.8 GHz, and has 6 MB of cache, whereas the i7 620M is a dual core clocked at 2.66 GHz which will Turbo Boost up to 3.33 GHz with 4 MB cache. The i7 620M will perform better in single core or dual core applications, while the i7 720QM would be better for multitasking.
The 720QM turbos up to 2.8/2.4/1.73/1.73 GHz when 1/2/3/4 are active, respectively.
The 620M turbos up to 3.33GHz when a single core is active, and up to 3.06 GHz when 2 cores are active.
-In single threaded applications: The 620 clocks at 3.33 GHz while the 720 clocks at 2.8 GHz > the 620 wins.
-In applications using 2 threads: 3.06 GHz (620) vs. 2.4 GHz (720) > the 620 wins
-In applications using 4 threads: The 620 turbos to 3.06 GHz (maybe a bit lower) while the 720 clocks at 1.73 GHz, so the 720QM : 4*1.73 vs the 620M : 2*3.06 (+ about 20% for hyperthreading), so the 620M may actually beat the 720QM even in applications that make use of 4 threads!!!!!!!
-In applications that make use of more than 4 threads: Hyperthreading kicks in (720QM) to give 8 threads, while the 620M can only give 4 threads, so the 720QM wins (maybe by 15-20%).
This was a rough (but useful) estimate, also the 720QM has a better memory controller and larger cache which may give it some advantage.
Also don't forget that with the 620M you don't need to have a discrete GPU (which increases power consumption), and the 620M itself also uses much less power than the 720QM, and it is faster in all but the most multithreaded applications, so unless you use multithreaded applications (like video editing) and don't care much about battery life, the 620M is the better choice.