BitLocker: Encrypting used space only or full space?

Used Disk Space Only encryption means that only data that is used now or is written in the future will be encrypted.

In your case, this is every bit as secure as encrypting the whole disk, since the disk is new and never had any data on it.

The reason that not ever having any data is important, is because sectors containing deleted files are not marked as used, so are not encrypted and can be read by forensic programs as plain-text. But as this is not your case, you are safe from this particular attack.