310-036 is an upgrade exam for those who have a prior
SCJP certification. 310-035 is the exam for those who are coming to SCJP for the first time.
The biggest difference between the two is that the upgrade exam does not ask you any questions on those things that have not changed from the prior version of the exam (in this case, the SCJP 1.2 exam). There are fewer questions and it's cheaper.
As for preparation... I cannot really state the usefulness of HFJ for the upgrade as I did not take the 1.4 exams - if the exams for SCJP 5.0 are any indication, the same prep materials are useful for both exams, if you factor in that you're only going to be asked the brand new material for the upgrade.
Hope this helps.