Based on other discussussions on the list my recollection is that IPSEC provides better performance if you need GigE or better data rates on your VPNs. My sense is that IPSEC may be more difficult to configure and less robust at keeping connections up, but this has probably improved in recent years.
ipsec is usually too complex .. for anything else than site to site tunneled connections.
The main advantage to pptp that I see is compatibility with mobile devices. A disadvantage of PPTP, as far as I know it cannot easily be tunneled through something like a linux firewall because it uses non-standard protocol packets (not TCP/UDP).
Well, at least linux support pptp connection tracking, but some poor firewalls do not.
-- Eero