I think the answer to your question depends on a couple of things:
Firstly, how much memory is going to be installed in the laptop? Adding more memory nowadays is going to give you a more noticeable boost in performance than processor speed. The speed difference between the two processors itself isn't going to be really that noticeable unless the installed RAM is quite low for the OS (even then you may hardly even notice it). In other words, a laptop with a 2.10 GHz processor and 2 GB of RAM is superior in performance to a 2.40 GHZ laptop with 1 GB of RAM.
Secondly, what are you going to use the laptop for? If you aren't going to be using it for processor-intensive tasks, a 2.10 GHz processor should be more than enough speed. For the price it will cost to upgrade I don't believe it would be worth it if the extent of your computing is internet browsing, word processing, and low end games. If you are going to use this for high end gaming, definitely get the processor upgrade and some more memory put in if you have less than 2 GB of memory in the system. If you are aiming for low end gaming, just upgrade the memory. Another thing to bear in mind: a dual core processor is going to give you better performance in most cases than a single core.
Lastly, you did not mention the hard drive specs, but I would advise getting one with the laptop that has least 120 GB of space. This is because sometimes manufacturers will make the hard drive a component difficult to upgrade. For instance, my Sony Vaio has no clear tear down instructions to swap the drive out and I therefore am stuck with an 80 GB drive with 20 GB used for OS installation backup files and about half of what's left taken up by windows and my installed programs. I still have about 35 GB left of storage but it's obvious I am not using this one as my main computer for storage.