Not all that much.
8MB drives are often a little faster, but not so much that you would
notice it without a benchmark.
There are a few exceptions. If use visual C++ you will notice it, and
strange enough also when you copy large ISO files and MP3s.
Xbitlabs did a nice review about it:
http://www.xbitlabs.com/articles/storage/display/4-maxtor-hdds.html
Most important reason to buy an 8MB disks nowadays is the extra
warranty. But I understand that Samsung also has 3 years on 2MB disks,
in which case it will just be a matter of price and availability.