Which processor is "better" kind of depends upon what you expect from your laptop. There is a web site you can use to review and compare the features & specs of these two processors side-by-side. Please see the link: http://ark.intel.com/Compare.aspx?ids=49022,43122.
In general, here are some ways you might differentiate the two processors you are trying to compare.
First off the dual core Intel Core i5-450M processor is intended for systems with integrated graphics. The CPU and graphics controller share the same package. The quad core Intel Core i7-720QM, on the other hand, is intended for systems with discrete graphics. If you are using graphics-intensive applications, then it is up to you whether integrated or discrete graphics will meet your needs.
Next is the number of cores & threads and cache memory size each processor features. The Core i7-720QM has double the cores, threads, and cache size of the i5-450M, which means the Core i7-720QM notice more overall performance depending upon the applications used. The trade off is that the i7-720QM (45W) requires more power than does the i5-450M (35W), so you may get better battery life with an i5-450M based system.
Please note though that battery life and performance are relative and are really dependant upon how a given system manufacturer optimizes their systems, so battery life and performance may vary between laptop vendors even when using the same processor.
Processor Number i5-450M i7-720QM # of Cores 2 4 # of Threads 4 8 Clock Speed 2.4 GHz 1.6 GHz Max Turbo Frequency 2.66 GHz 2.8 GHz Cache 3 MB Intel® Smart Cache 6 MB Intel® Smart Cache
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