PPTP vs. L2TP

PPTP use two port

TCP/1723 to allow the PPTP control traffic, such as LCP, CHAP and most PPP option.
then use IP protocol 47 (Generic Routing Encapsulation) to move data. PPTP without MPPE is not secure. Actually MPPE is used in GRE to encrypt the data.

L2TP

From the Figure 3.0, actually there is not encrypt on L2TP. So when it use UDP as packet transport, it can be used over IPSec.

PPTP and L2Tp are layer 2 tunnel. GRE and IPSec are actually layer 3 tunneling.

Q. Do I need L2TP if I already have IPSec?
A. L2TP is a standard for Layer 2 tunneling. IPSec is a standard for encryption and security. They are independent, yet complementary standard efforts, and Cisco will provide the combination to take advantage of the strengths in both technologies. Strengths in L2TP include per-user authentication, dynamic address allocation from an address pool or by using DHCP server, and RADIUS and AAA support. Some of the strengths in IPSec are secure encryption and data confidentiality.