Does using the new FileVault use up less disk space than Legacy FileVault? I was using (Legacy) FileVault on Snow Leopard and when I upgraded to Lion, it kept me on Legacy FileVault. I notice that FileVault uses a lot of disk space which is then recovered once I log out. Sometimes this can be in the range of 5-8 GB. I'm thinking about switching to the new FileVault in Lion, but I'm wondering how the disk usage is compared to Legacy FileVault.

Does the new FileVault in Lion use up more or less space than Legacy FileVault? Will it continue to use up a lot of space temporarily (i.e. after a restart it will free up all that space)?
The two technologies share nothing except the name and cannot be directly compared.  The old Filevault created a spare bundle image file for your home folder, which was encrypted.  The new one simply encrypts the whole drive in a way that is invisible to the user.

If anything there may be a slight overheard to managing a sparsebundle which will not be there for whole disk encryption, but I doubt that it will be significant enough to make it a part of your decision to use it or not.