I run an SRM and have used the following head units, SRM PC7, Garmin 500 and Garmin 705.
The 705 I used briefly, great readout, easy to read and tons of info on one page.
Benefit of the 705 is if you have bad eyes its much easier to read.
The Garming 500 is what I use the most, its faster than the 705 (boot time etc..) great battery life and almost as much info as the 705. However if navigation is important to you then the 705 is the way to go. The 500 screen is way to small to use for mapping.
Downsides of the 500 are... lockups and data loss.. I have had three out of 100 or so rides.
If you read the 500 forums other users have had the same issue. Only one ride I lost all my data and that one happened to be an LT test for my coach so I was really pissed.
For simplicity the PC7 is the best.. you get power, cadence, speed and time on the display and easy to press buttons for splits. I alternate between the PC7 and 500.. I use the edge for long rides since you can review the exact sections the road that struggled or hammered on.