Having spent about equal hunting time out of each, I sort of agree with
@sb220 that climbing method has an impact. The build in lineman's loops of the kestrel work great with a stick or step climbing method. The belay loop of the RC harness works nice with rope climbing techniques.
I found a SitDrag was easier to find comfort with initially, but once I figured out how to best adjust my Kestrel it's a horse a piece. Both are comfy.
I feel more connected to the Kestrel than the a sling because it's directly attached with both the waist and leg loops. With the SitDrag, I sometimes felt the sensation that I was going to slide out the bottom. And if I ever stood on a platform, putting slack in the tether, the sling felt more loose, whereas the Kestrel doesn't move.
Kestrel is easier to go to the bathroom with and has the molle webbing. You can add webbing with a sling (haven't done it with fleece) so in regards to accessories it's just a difference of saving money and adding them manually with the DIY route vs having everything already done for you with the Kestrel.
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