Does the alternate installer's "Guided - use entire disk and set up encrypted LVM" use LUKS or plain dm-crypt?
The encryption uses LUKS with dm-crypt (not plain dm-crypt).
It's more likely that Ubuntu uses the recommended and more secure way of disk encryption. Now, to verify so, read the contents of the disk (assuming that /dev/sda2 contains your encrypted partition):
You'll see several options, such as the cipher being used and the ID of the encrypted partition.
Side note: if you only have one partition to encrypt, I suggest you to avoid LVM at all and use LUKS only (which can be done with the manual partitioning method). This avoid the overhead of LVM which you don't need for just a single partition. On the other hand, if you have multiple partitions to encrypt (/, /home, swap), LVM on the top of LUKS is more convenient as you have to enter your passphrase only once. (physical - LUKS - LVM - /home, /, swap, etc)