Openvpn is considerably more cpu intensive.
PPTP uses GRE to transport the data sometime this is hard to get though some routers. If you run the vpn on the routers themselves then it is not a issue.
It has been a long time since I used PPTP and forget how it actually encrypts. I know it is not considered secure.
If you don't care about security at all then just use a simple GRE tunnel. All it does it wrap the packets with a second header it does not change the payload. I know GRE Is supported by commercial routers and DD-WRT but I don't know about other router implementations.