I went with the seekins on my 6.5cm because it uses a "button" style release instead of the lever style. I use the 6.5 for hunting and just liked the idea of having fewer things protruding from the rifle. Less for things to get caught on.
An interesting thing with my build is that i used a timney flat trigger and with the seekins bottom metal, just the shape of the trigger guard and the length of the trigger, .030-.040" had to be shaved off of the bottom of the trigger for it to clear.
I have no experience with the APA botom metal.