Which type of VPN

Hi everyone

I run hotspots at different locations each with its own DSL connection and Mikrotik router. I have a radius server(Radius Manager 4) that all my hotspots connect to via PPTP to authenticate users.
Overview of setup:

Radius Manager - Mikrotik(PPTP server) - DSL modem - Internet - DSL modem - Mikrotik(PPTP client) - Hotspot users

I have been reading up a bit and it seems that there are some possible security flaws with this method.
Can anyone suggest a better method for me to connect my Mikrotik routers at each hotspot to my main Mikrotik router so that users can be authenticated?

You could easily switch to L2TP.

But there are plenty of options.
http://wiki.mikrotik.com/wiki/Category:VPN