As far as I understand PAC supports both rdesktop and xfreerdp as RDP back-ends, but chooses rdesktop over xfreerdp, when both are installed. My experience working with both back-ends is, that rdesktop works better with old Windows clients, like Server 2003, XP and below and xfreerdp works better with new Windows clients, like Server 2008, 7 and above (and if these machines are configured with enforced security via NLA/certificates, using xfreerdp is mandatory).
So my feature suggestion is to support both (well, already there) and let the user choose which connection back-end to use on a per connection basis. A simple ComboBox on the RDP Options tab should do the trick. Options should be the lowest common denominator of all options between the two back-ends (as it is now), and a TextBox that appends extra options on a command line basis should be sufficient for special options.
Thanks for your time and effort you are putting into this great connection manager software!