Here are some points of contrast. These are based upon my technical understanding of both apps, however I have not used Xtralogic's app. These are just facts, and their evaluation is subjective (up to you).
Connectivity
* RDP will require that you open holes in your firewall and forward ports.
* RDP will require that you know your PC's IP address, and change it when/if your DHCP lease changes.
* PhoneMyPC will perform best if you can configure your network, but it is not required for the app to work.
* PhoneMyPC is a hosted solution, so you don't need to know your IP--it just works.
Performance
* RDP uses a vector-based graphics protocol that often requires less bandwidth, but more CPU on the phone. You are likely to get better performance when network bandwidth is low.
* PhoneMyPC uses bitmap-based graphics data that can consume more bandwidth, but is very light-weight on the phone in terms of CPU workload.
Quality
* Most RDP clients remove things like desktop wallpaper because they do not handle bitmap data well.
* PhoneMyPC shows you your desktop as it is, with no changes or compromises.
Features
* RDP supports remote login.
* PhoneMyPC does not, but will in the next few weeks.
* RDP supports sound.
* PhoneMyPC does not, but will very soon.
* PhoneMyPC has many features that RDP does not, such as:
- Passive live viewing
- Snapshot on the phone
- Direct Process and Window management (handy for uber geeks, particularly when fighting viruses)
- Control-pad: ideal for using the phone as a simpler remote while giving presentations, changing channels on your Media PC, etc.
- Action list: easy way to perform common, pre-programmed actions on the PC.
Those are the facts.
As for opinions, I'd say that RDP is Microsoft's long-standing, mature remote-access technology; it's reliable and simple, and a great choice for geeks.
PhoneMyPC is a new approach, more fully featured (though temporarily missing a couple important ones) but also less mature and constantly evolving (improving).
My best suggestion would be to download both and give them a try. You can return either or both within 24 hours through the market.
Good luck!
SoftwareForMe.com
Makers of PhoneMyPC